Launch one small product, end to end

Write the customer contract in plain language

Specify the accepted file format, maximum size, report contents, processing expectation and what happens to rejected rows. Decide how long inputs and results are kept, whether users can delete them and what they can export. Show these limits before upload.

Prove the report with realistic sample data before adding tiers or an AI feature. Keep a run ID and visible states such as queued, processing, ready and failed. A failed run should leave the input available under the stated retention policy and explain how to retry without creating duplicate work.

Separate customer data at every entry point

Associate every file, job and report with its owner or workspace. Check authorization when reading, changing, exporting and deleting records. An obscure URL is not an access policy.

If using Supabase’s Data API, its guidance calls for Row Level Security and suitable policies. Secret and service-role keys bypass that protection and must stay on the backend. See Supabase: securing data. Test the selected policies through your actual application and storage paths, including any privileged worker.

Next.js and Vercel are framework and hosting candidates. Check upload and execution limits for the chosen configuration. Move processing to a durable job mechanism if the work cannot finish reliably within the request path; do not silently promise completion after the request has failed.

Make billing and access agree

Define your entitlement policy first: what a trial can do, how a failed payment affects new jobs, and when cancellation ends paid access. A checkout return page alone should not unlock an account.

Lemon Squeezy documents subscription events for synchronizing local records, and an X-Signature check for webhook authenticity. See Lemon Squeezy: subscription webhooks. Verify the raw request body and signing secret before processing an event. Record the accepted event and apply changes durably before acknowledging success.

Make repeated delivery safe, reconcile local entitlement against provider state and rehearse failed processing. Keep a cancellation request distinct from the date access actually ends under your policy. Show the customer the resulting state and give support a way to explain it.

Run a customer boundary check

Customer A knows customer B’s report ID

Reading, downloading, changing and deleting the report are denied across both the application and storage paths.

The same upload or event arrives twice

One intended report or entitlement change results; the repeat is recognized rather than charged or applied again.

Payment fails or a subscription is cancelled

The displayed status and permitted jobs follow the documented policy, including any paid-through date.

Processing stops after a deploy

The run is recoverable or explicitly failed. The customer can see the outcome and the operator can diagnose it.

A customer leaves

They can export the promised data, understand retention and request deletion through the supported path.

Price the service you can operate

Estimate hosting, database and file storage, processing, email, payment-provider charges and support at your expected usage. Add model usage only if the report needs it. A free tier is a starting configuration, not a guarantee of profitable recurring service.

Run the full path with a separate customer account and payment-provider test mode before accepting a real payment. These are proposed checks, not a claim that this example has been deployed or benchmarked.