You will get a payment gateway integrated end to end: 3DS, webhooks, reconciliation
Top Rated
Project details
One payment provider integrated into your product, end to end: card processing with 3D Secure, webhooks for async status, refunds and cancellations, idempotency so retries never double-charge, and reconciliation so your records match the provider's.
I do this work daily on a marketplace payments platform moving about $4B a year. I have integrated Adyen, Braintree, Payoneer, PayPal, Wise and bank vPOS systems, plus Stripe and FastSpring in client products.
You get working code in your stack (Java/Spring Boot or Node.js), tests, and documentation of the flows.
I do this work daily on a marketplace payments platform moving about $4B a year. I have integrated Adyen, Braintree, Payoneer, PayPal, Wise and bank vPOS systems, plus Stripe and FastSpring in client products.
You get working code in your stack (Java/Spring Boot or Node.js), tests, and documentation of the flows.
Programming Languages
JavaScript, Java, TypeScriptCoding Expertise
Localization, Performance Optimization, SecurityWhat's included
| Service Tiers |
Starter
$2,500
|
Standard
$4,500
|
Advanced
$8,000
|
|---|---|---|---|
| Delivery Time | 21 days | 30 days | 45 days |
Number of Revisions | 1 | 2 | 4 |
Design Customization | - | - | - |
Content Upload | - | - | - |
Responsive Design | - | - | - |
Source Code |
Frequently asked questions
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AC
Alexis C.
Jul 27, 2026
Lead Backend Developer
AA
Aleksandar A.
Jun 29, 2026
Senior Java Developer Needed for Payment Gateway API Integration
We loved working with Oleg. The type of developer that is detailed-oriented, doesn't rush into things and what he delivers doesn't need someone to go over it and fix it. We are definitely working with Oleg again.
AT
Alfiya T.
Mar 23, 2026
JavaScript/Angular/React Developer for API Demos and Code Samples
Working with Oleg has been a very positive experience. He supported us across multiple areas, including library development, code samples, technical articles, and product maintenance, and consistently delivered high-quality results.
What stood out most was the combination of speed and reliability. Communication was always clear and efficient, and tasks were handled quickly without compromising quality. Thanks to his expertise, we were able to save a significant amount of time and accomplish more than expected.
He is highly responsible, easy to work with, and offers a great balance between cost and quality. Overall, a very reliable professional — I would definitely recommend working with him.
What stood out most was the combination of speed and reliability. Communication was always clear and efficient, and tasks were handled quickly without compromising quality. Thanks to his expertise, we were able to save a significant amount of time and accomplish more than expected.
He is highly responsible, easy to work with, and offers a great balance between cost and quality. Overall, a very reliable professional — I would definitely recommend working with him.
AC
Alexis C.
Jan 26, 2026
Lead Backend Developer
AC
Alexis C.
Jul 28, 2025
Lead Backend Developer
About Oleg
Senior Backend Engineer | Payments & AI Agents | Java, Spring Boot
100%
Job Success
Yerevan, Armenia - 12:16 am local time
PAYMENTS AND FINTECH
I deliver provider integrations end to end: Adyen, Braintree, Payoneer, PayPal, Venmo, Wise, Wells Fargo, Bank of America and Forter, on a platform operating across 180+ countries. My work covers card processing, wallets, payouts, bank transfers, fraud screening and reconciliation over REST, SFTP and webhooks. I have also integrated Stripe, FastSpring and bank vPOS in client products.
Representative work:
- Migrated a major payout provider from a legacy protocol to REST with OAuth 2.0. The migration covered payouts, mass payouts, payee registration, card release and reconciliation. I shipped it to 100% behind feature flags with no customer-facing incidents. I built provider emulators to test it without vendor sandboxes.
- Designed a card-processing service on a provider's GraphQL API covering client tokens, payment methods, network tokens, cryptograms, authorization, capture, refunds, cancellations and 3D Secure.
- Built a forwarding layer that routes network tokens between two providers, enabling non-US card acceptance. I rolled it out in stages and negotiated timeout SLAs directly with the vendor.
- Authored the design for splitting a payments monolith into nine provider-specific microservices.
Domain coverage: 3DS2, network tokenization, cryptograms, authorization and capture, refunds, chargebacks, reconciliation through APIs, webhooks and SFTP files, idempotency and double-charge prevention, mass payouts, ACH returns, fraud screening, PCI-aware design.
AI AGENTS AND LLM SYSTEMS
I build AI that runs inside products with real users and revenue.
I built an agentic system that investigates failed payment transactions on its own, reasoning across logs, dashboards, source code and incident tooling to cut manual triage. I built it with Python, LangChain and multi-agent orchestration. It runs in a high-stakes payments environment, where being wrong has a cost.
I also built and run Memabooks, a profitable AI commerce product that turns a child's photo into a personalized illustrated storybook. Over 10,000 books created so far, printed on demand and shipped worldwide, with customers in eight countries.
I developed a multi-stage, identity-preserving image generation pipeline covering face detection, background removal, personalized character generation, skin-tone matching and character consistency across a complete book. I work with diffusion models, fine-tuned LoRA adapters and serverless GPU inference, tuning for quality, latency and cost.
I also built the Node/NestJS backend, the Angular front end, print-on-demand fulfilment, and the payment stack for a market with limited access to major global processors: bank vPOS, 3DS2, Apple Pay, idempotency and double-charge prevention, and abandoned-payment recovery.
AI stack: LangChain, LangGraph, LlamaIndex, multi-agent systems, RAG and multimodal RAG, vector databases (FAISS, Chroma), Model Context Protocol, OpenAI, Gemini, Azure AI.
BACKEND ENGINEERING AND RANGE
My core stack is Java 17 and Spring Boot. I also have production experience with Node.js, NestJS, TypeScript, Python, PostgreSQL, MongoDB, AWS, Docker, Kubernetes, REST, GraphQL, microservices and event-driven architecture.
I was the sole engineer responsible for Geoapify's product surface for 17 months, working directly with the product owner across 14 repositories. I built a geocoding SaaS end to end, covering file upload, worker-based job processing, object storage and card payments. I maintained their public TypeScript SDKs for MapLibre GL, Leaflet and GeoJSON, which see about 55K npm downloads a month, and owned their public API playgrounds and OpenAPI specs.
Testing is part of my delivery process. On one project, I built 260 Playwright end-to-end tests from scratch in about six days.
WHAT I CAN HELP WITH
- Payment provider and fintech integrations
- Java and Spring Boot backend development
- Backend architecture and monolith-to-microservices migrations
- AI agents, RAG and LLM integration
- API design, reliability, observability and production troubleshooting
CREDENTIALS
- Oracle Certified Professional: Java SE 11 Developer
- IBM RAG and Agentic AI Professional Certificate
- Google Cloud Generative AI Leader
- Microsoft Azure AI Fundamentals
- BSc in Computer Science, American University of Armenia
- Former Java lecturer, Russian-Armenian University
My longest engagement here ran 19 months and 2,700+ hours. I am open to long-term, full-time and contract-to-hire engagements.
Tell me what you need built or what is currently breaking. I will tell you honestly whether I am the right engineer for it.
Steps for completing your project
After purchasing the project, send requirements so Oleg can start the project.
Delivery time starts when Oleg receives requirements from you.
Oleg works on your project following the steps below.
Revisions may occur after the delivery date.
Scope and access
We confirm the provider, flows and tier scope from your requirements. You give me repo access and provider sandbox credentials.
Integration build
I build the payment flows in your stack against the provider sandbox: payments, webhooks, refunds, idempotency. You see progress in the workroom.