How I'd build Collect AI
I'd reach for Next.js on the frontend, FastAPI on the backend, and Postgres for data. Stripe for billing, Resend for email because their reputation is clean and that solves the deliverability risk immediately, and direct API integrations with QuickBooks, FreshBooks, Xero, and Stripe for the sync layer. Rough estimate: 350-400 hours to launch, so about 8-10 weeks at a sustainable pace. That's 400 hours at your budget, so we're looking at the full allocation.
Day-by-day plan
Day 1-2: Postgres schema for tenants, users, invoices, and payment history. Stripe customer objects wired to tenant accounts. Multi-tenant isolation at the DB level.
Day 3-4: Implement the three pricing tiers in Stripe. Test upgrade/downgrade flows and proration. Wire billing into the auth layer so tier limits are enforced in the UI.
Day 5-6: Build the onboarding flow: email verification, company name capture, accounting platform selection. Store OAuth credentials securely in a vault, never in plain text.
Day 7: Core dashboard: invoice list, filter by status (pending, paid, overdue), basic search. Wire the display layer to the integration data.
Day 8-10: QuickBooks integration. OAuth handshake, invoice sync from QBO into Postgres, handle rate limits and incremental pulls. This is the riskiest piece.
Day 11-12: FreshBooks and Xero via their REST APIs. Test credential rotation, handle revoked access, test the reconnection flow.
Day 13: Stripe integration for syncing recent charges as invoices. Handle the case where a charge is already in our system.
Day 14-15: Resend setup for transactional email. Build templates for payment-due reminders, overdue alerts, payment-received confirmations. Set up DKIM/SPF verification, test deliverability with GSuite and Outlook.
Day 16-17: Scheduled reminders via background jobs. Define rules: send reminder 3 days before due, send overdue alert 5 days after. Wire into Celery with Redis for task state.
Day 18: Testing across all integrations. Data consistency checks. Edge cases: partial payments, refunds, invoice deletion, what happens when a user disconnects their accounting tool mid-sync.
Day 19: Deployment to AWS. Set up RDS Postgres, Lambda for background jobs, CloudWatch monitoring. Seed test data from real accounting platforms.
Day 20: Documentation, Loom walkthrough, runbook for troubleshooting failed syncs.
What's hard about this build
The integration layer is the real risk. Each accounting platform has different rate limits, different failure modes, and their OAuth tokens expire unpredictably. QuickBooks in particular has a 60-minute auth token lifetime with refresh token rotation that'll catch you off guard. You also need to handle the case where a user disconnects their accounting tool midway through a sync, or deletes an invoice that we've already pulled - that creates orphaned records and reconciliation headaches. Silent failures are worse than loud ones: if invoice syncs quietly stop working, you won't know until your first customer's payment deadline passes and they're asking why reminders never arrived. The other piece: if the integration takes longer than expected to stabilize, your churn happens before you fix it. These businesses operate hand-to-mouth and need their integrations working on day one, not week two.
What's fast because of AI
AI accelerates the scaffolding dramatically. I'd use Claude to generate the FastAPI router stubs for each accounting platform, the Postgres migrations, and the Stripe webhook handlers. That's a week of tedious boilerplate that Claude generates in an afternoon. Testing is faster too: prompt Claude with the integration logic, ask it to enumerate edge cases, and it'll catch the partial-payment and refund scenarios you'd miss. Email copywriting for the reminder templates gets polished fast: seed the copy with Claude, iterate on tone until it feels urgent without looking spammy. And debugging: when a Stripe sync fails at 2am, Claude can help trace through the webhook logs and generate hypotheses about what broke, saving hours of manual inspection.
How I'd hand it off
I'd record a Loom walkthrough of the admin panel, the settings for integrations, and how to rotate OAuth credentials. Leave a runbook in Notion: "If QuickBooks sync is failing, check (1) token expiry, (2) rate limit logs, (3) invoice count drift." 30-day pager rotation: you page me if churn spikes or syncs go silent for more than an hour. I'll transfer all credentials to your vault, set up 1Password sharing for the team, and leave admin access to Stripe, Resend, and AWS. First production incident, I'm on the call with you to triage it.