You have the idea. I make it billable.

• Accounts and organisations — proper multi-tenant separation, enforced by the database rather than by remembering to filter.
• Payments — Stripe checkout for one-off bundles and subscriptions, with the merchant-of-record route so VAT filing is not your problem.
• A credit ledger that holds up — append-only, so a duplicate webhook grants credits once and a crash mid-purchase cannot strand someone's balance.
• VAT and business checks — tax codes, VAT-number validation, and the choice between selling to businesses only or opening up to consumers made deliberately rather than by accident.
• The decisions, written down — what was chosen, what was rejected, and what would change the answer. You get the reasoning, not just the code.

I built this layer for my own products. Along the way I found out — by paying real money into a sandbox, not by reading documentation — that a payment provider's "business use" tax code does not stop a consumer from buying, that requiring a VAT number is rejected outright on the merchant-of-record route, and that a webhook you thought was handled correctly still needs proving with a second delivery.

That is the part worth paying for. The code is a week; knowing which of those decisions bites you in month three is what actually costs.

Honest scope: this is the billing and account layer around your product. It is not your product, and I do not pretend to know your domain better than you do.