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Tina Diet Docs

Overview

Tina Diet’s premium tier is 150 THB for 30 days, paid via Omise PromptPay QR or TrueMoney Wallet. No auto-charge — users pay each period explicitly, and unspent days carry over via grant stacking.

Why this model?

In Thailand:

  • PromptPay is universal (every bank’s mobile app supports QR scan)
  • TrueMoney Wallet is the second-most-popular e-wallet
  • Recurring auto-charge is much rarer than in the West; Thai users are accustomed to one-time purchases and explicit renewals
  • Stripe-style auto-renewing subscriptions don’t fit local payment habits

So we use Omise’s one-time Charges API (not their Recurring Schedule API), and treat each successful charge as a 30-day grant.

Grant stacking

When a payment succeeds, services/omise.ts computeGrantWindow() computes:

const stackStart = currentExpiryMs > now.getTime() ? currentExpiryMs : nowMs;
const grantEnds = stackStart + grantDays * 24 * 60 * 60 * 1000;

So:

  • If user pays while expired (or never paid): new expiry = now + 30 days
  • If user pays while still premium: new expiry = current_expiry + 30 days

The user never loses days. They can buy two months ahead and the expiry pushes 60 days from current.

LIFF mirrors this math in lib/premium.ts computeProjectedExpiry() so the “วันหมดอายุใหม่จะเป็น 11 ส.ค. 2569” preview shows the correct date BEFORE the user pays.

Payment provider strategy

ProviderStatusReason
Omise✅ ActiveNative Thailand provider, lower fee (~3.65% vs Stripe’s 4.65%), supports PromptPay + TrueMoney natively
Stripe💤 DormantCode preserved (services + routes), env empty → graceful 503. Worth keeping for future SaaS subscription model

The subscriptions table is Stripe-only; the payments table is Omise-only. This keeps semantics clean: Stripe = ongoing relationship, Omise = discrete transactions.

Methods supported

MethodUXBackend source.typeAuto-renew?
PromptPay QRIn-LIFF modal with QR + countdown + pollingpromptpay❌ Manual each time
TrueMoney WalletRedirect to Omise authorize page → TrueMoney app deep-linktruemoney_jumpapp❌ Manual each time

NOT supported (could be added):

  • Credit/Debit Card (would need Omise’s Schedule API for true auto-renew — scoped out of v1)
  • Internet banking (less popular than PromptPay)
  • Rabbit LINE Pay (deferred — overlap with LINE ecosystem might confuse)

Idempotency

Payments table has UNIQUE (provider, provider_charge_id). Webhook handler in services/omise.ts handleOmiseEvent() checks payment.status === 'successful' and early-returns if already processed.

Same event delivered twice (Omise retries on non-2xx, sometimes delivers duplicates) is safe — second delivery is a no-op.

Webhook flow

PromptPay scan / TrueMoney auth
  │
  ▼
Omise records payment
  │
  ▼
Omise fires charge.complete webhook
  │
  ▼
POST /webhooks/omise on backend
  │
  ├──→ verifyOmiseSignature() against Omise-Signature header
  │     (HMAC-SHA256 of `${timestamp}.${rawBody}` using base64-decoded secret)
  │
  ├──→ parseOmiseWebhookEvent() validates shape
  │
  ├──→ Respond 200 immediately
  │
  └──→ Async handleOmiseEvent():
        - Find payment row by charge_id
        - If status already 'successful', return (idempotent)
        - Otherwise: mark successful, compute grant window, applyPremium on user

See Webhook signature for the HMAC details (and the lesson learned from initially assuming Basic Auth).

Polling fallback

Webhooks can be delayed or missed. LIFF polls GET /api/v1/billing/omise/charge/:id every 2 seconds while the PromptPay modal is open. If still pending AND Omise is configured, the route also calls syncChargeFromOmise() which retrieves the current state from Omise directly and applies grant if successful.

This means even if the webhook never arrives, the user still gets premium extended within a few seconds of the poll detecting success at Omise.

Going live

TEST mode (currently active) lets us fully simulate flows without real money:

  • TEST pkey_test_* + skey_test_* keys from Omise dashboard
  • PromptPay QR rendered with “TEST MODE” overlay
  • TrueMoney redirects to Omise’s authorize SIMULATOR page (3 buttons: Successful / Failed / Pending)
  • Webhooks fire normally

LIVE mode requires:

  1. Omise business verification approved (Thai docs: ภพ.20, DBD, bank, ID)
  2. PromptPay + TrueMoney methods enabled in Omise dashboard
  3. TrueMoney provider approval (separate, +7-14 days lag possible)
  4. LIVE API keys (pkey_* + skey_* without _test_) set in Railway env
  5. LIVE webhook secret in Railway env

No code change between TEST and LIVE — just env values.

See Omise integration for the full integration details.

Cost model

ItemCost
Pricing150 THB / 30 days
Omise fee~3.65% (Thai cards/PromptPay/TrueMoney)
Net per payment~145 THB
OpenAI marginal~$0.004/user/day max if cap (20 questions × $0.0002)
Net margin~140 THB / user / month at full premium quota use

When TrueMoney has higher fees (e.g. specific bank), check with Omise rep for exact rates.

Future ideas

  • Annual plan with discount — would need different grant_days per product
  • PromptPay X / TrueMoney Pro tiers if usage data justifies
  • Recurring auto-charge via card if user demand emerges — would re-enable Stripe (since Omise’s Schedule API has Thai-market quirks)