Full-Stack Developer QBO Payment Sys + 3rdparty API (Customer Intake - Automated Billing)

IMPORTANT: We are strictly looking for an individual, dedicated contractor. No agencies, development firms, or subcontracting will be considered. Currently acquiring an exclusively dedicated developer with strong US time zone familiarity. As a software firm, we are looking for a reliable partner who can manage several projects, including direct customer management and clear communication. Fluent English is a high value and strictly required. Please note: If you are currently full-time engaged in another local job or cannot dedicate focused, overlapping hours to this project, this role is not for you. We need someone who can treat this as a primary priority. Project Overview: We are looking for a talented full-stack developer to redesign/update a website (currently hosted on a standard domain registrar) and build a custom, multi-signer intake form with secure document upload capabilities. The core of this project is integrating direct, custom backend automations with a third-party Remote Online Notarization (RON) API and QuickBooks Online. No third-party automation tools (Zapier, Make, etc.) are permitted. All integrations must be built via custom, standalone backend code. - Key Deliverables: Frontend Intake Form: Clean, mobile-responsive design allowing secure file uploads and dynamic form fields based on the input signer count. Direct Third-Party API Integration: Connect to the RON provider’s REST API to instantly initiate a session and securely pass files upon customer submission. QuickBooks Payments SDK Integration: Implement secure credit card vaulting/tokenization at intake using the official QuickBooks Web Payments SDK (ensuring zero PCI-compliance liabilities on our end). Admin Post-Session Billing Tool: Create a lightweight, secure, password-protected internal portal where the admin can input final signer and seal counts after a session concludes. Automated Invoicing Engine: Calculate the final dynamic pricing via custom code upon admin submission, generate a line-item invoice in QuickBooks, automatically charge the vaulted card token, and trigger the official receipt to the client. - Technical Requirements: Proven experience with backend languages (Node.js, Python, PHP, or Go). Deep familiarity with QuickBooks Online Accounting & Payments APIs (specifically tokenized card charges, OAuth 2.0 token refresh cycles, and invoice generation). Strong understanding of REST APIs, webhooks, JSON payloads, and secure file handling over HTTPS. Ability to deliver clean, documented, standalone code without external platform dependencies (e.g., deploying a lightweight containerized solution to Render, AWS, or similar). How to Apply: To ensure you have read this entire post, please start your proposal with the word "DEDICATED". - In your proposal, please include: A brief example of a similar custom API integration you have built (specifically involving payment gateways or document workflows). - FYI We place great importance on honesty. Even if you lack relevant experience, welcome anytime provided you possess confidence and a solid technical background. Your approach to managing the QuickBooks OAuth token refresh cycle to ensure the chain never breaks during server restarts. Your estimated timeline and fixed-price or hourly rate for this scope.