QuickBooks Online Payment Portal Integration

We are a service business using QuickBooks Online (QBO) for customer invoicing and recurring monthly billing. We are looking for an experienced developer to create a simple, professional customer payment portal accessible through our existing Squarespace website. Our primary goal is to make it easier for customers to view and pay their outstanding QuickBooks invoices online, while keeping QuickBooks Online as our accounting system and system of record. We are specifically looking for someone with experience working with the QuickBooks Online / Intuit API, OAuth 2.0, QuickBooks invoices, and online payments. Ideally, a customer would visit a "Pay Invoice" or "Client Portal" page on our website and be able to securely access their account. Once authenticated, we would like the customer to be able to see information such as: Current outstanding balance Open invoices Invoice number Invoice date and due date Invoice amount Payment status Ability to select an invoice and securely pay it Previously paid invoices, if practical Ability to enroll in or manage AutoPay for recurring invoices, if supported by the QuickBooks/Intuit platform The actual handling of credit card or bank account information should remain with QuickBooks Payments/Intuit or another PCI-compliant hosted payment solution. We do not want sensitive payment information stored directly on our Squarespace website. QuickBooks Integration Our company currently uses: QuickBooks Online QuickBooks recurring invoices QuickBooks Payments ACH/bank payments and credit/debit card payments We would like QuickBooks Online to remain the source of truth. Payments made through the portal should properly correspond with the appropriate QuickBooks customer and invoice so that our accounting staff does not have to manually reconcile portal payments whenever possible. Website Our existing website is built on Squarespace. We are open to recommendations regarding the best architecture. We understand that the actual customer portal may need to be hosted separately and linked to or integrated with our Squarespace website. We are not requiring that the entire application technically run inside Squarespace if a separate secure web application is the better solution. Scope / Deliverables We would like the selected developer to: Review our current QuickBooks Online and QuickBooks Payments setup. Recommend the simplest and most reliable architecture for the portal. Configure the necessary Intuit/QuickBooks developer application and OAuth connection. Connect the portal to our QuickBooks Online account. Implement a secure method for identifying/authenticating customers. Retrieve the appropriate customer's outstanding invoices from QuickBooks. Display those invoices in a clean, mobile-friendly customer interface. Allow customers to securely initiate payment of an invoice. Ensure payments are properly associated with the appropriate QuickBooks invoice/customer. Integrate or link the portal with our existing Squarespace website. Test the complete workflow on desktop and mobile. Provide basic documentation so our staff understands how the integration works. Deploy the finished portal/integration. Important We are looking for a simple, dependable solution, not a large custom software project. If some of the functionality above can be accomplished more effectively using QuickBooks' existing hosted payment functionality rather than custom development, we are open to that recommendation. Our priorities are: Easy for customers → Secure → Reliable → Properly synced with QuickBooks → Easy for our staff to maintain. Required Experience Please apply if you have experience with several of the following: QuickBooks Online API Intuit Developer Platform OAuth 2.0 QuickBooks Payments QuickBooks invoice/payment integrations Customer portals Payment processing integrations REST APIs Squarespace integrations Secure customer authentication PCI-compliant payment workflows When Applying Please include: Examples of QuickBooks Online API integrations you have previously built. Whether you have specifically worked with QuickBooks invoices and/or QuickBooks Payments. A brief explanation of how you would approach this project. Whether you recommend building a custom portal or leveraging QuickBooks-hosted payment pages for part of the workflow. Your estimated project cost. Your estimated timeline. Any ongoing hosting or software costs we should expect after launch. Please begin your proposal with the words "QBO PORTAL" so we know you have read the complete project description.