Full Stack Developer / Front End Developer for a written billing and seats scoping

We run a B2B SaaS with US-based customers, and until now every subscription has been handled by our team by hand - a quote, an invoice, and a spreadsheet of who has how many seats. That does not hold any more. We want customers to buy, add seats and upgrade on their own, and we want a written scoping document from a senior full stack developer before we commit to the build. What the scoping document should cover: - the plans, and what each plan actually unlocks inside the product - seat management - inviting, removing and re-assigning seats, and what happens mid-cycle - checkout, trials and upgrades, including how proration is handled - invoices, tax and failed payments, including what the customer sees when a card fails - what our own team still has to be able to override, and where that lives - the order to build it in, and what should deliberately wait We are leaning React with Next.js on the front and a Node service behind it on top of Stripe, and the billing admin would sit in the ops console we are already building rather than in a separate tool. We want a senior opinion in writing before we lock any of that. A front end developer view on the self-serve screens is part of it, but the harder half is what happens to a customer's access the moment a payment fails. The product is live, so our current plan structure, customer numbers and the internal spreadsheet go to the person we hire, under NDA - not before. Nothing needs to be accessed to apply. To apply, please share: - 2-3 subscription or billing surfaces you have shipped - one thing teams usually get wrong when they move from manual invoicing to self-serve - your fixed price for the scoping document