Add updateVatlyCustomer() method

[sandervanhooft]

Summary

Add methods to update customer details at Vatly.

Current State (March 2026)

✅ Billing Address Updates — Available

Vatly uses a redirect flow for billing updates (more secure, handles payment mandates properly).

Existing endpoint:

PATCH /subscriptions/{subscriptionId}/update-billing

Returns a hosted checkout URL (30-min expiry) where customer can update billing details.

SDK can wrap this — return the redirect URL for the merchant to send to their customer.

🔄 Payment Method Updates — In Progress

Tracked in Vatly core: #1004

Will provide a signed route (non-expiring) similar to customer invoice links:

/customer/update-payment/{customer}?signature=xxx

SDK can generate/wrap this once available.

⏸️ Direct Field Updates (email, locale, metadata) — Not Planned

No direct PATCH /customers/{id} endpoint exists. Updates flow through checkout for security/consistency.

If there's demand for simple field updates (email changes, metadata), a lightweight endpoint could be added to Vatly core.

Revised Acceptance Criteria

• API: Billing update via redirect flow (exists)
• API: Payment method update URL (#1004)
• SDK: requestBillingUpdate(subscriptionId) → returns redirect URL
• SDK: requestPaymentMethodUpdate(customerId) → returns signed URL (pending #1004)
• Documented in README

Notes

Vatly's architecture intentionally uses redirect flows rather than direct mutations for billing/payment updates. This ensures proper mandate handling and security.

[sandervanhooft]

Updated based on Vatly core investigation (March 2026):

Key insight: Vatly uses redirect flows for billing/payment updates, not direct PATCH mutations. This is intentional — it handles Mollie mandates properly and is more secure.

What the SDK can do now:

• Wrap PATCH /subscriptions/{id}/update-billing to return redirect URL

Waiting on:

• Payment method update URL (sandervanhooft/vatlify#1004)

Removing the 'blocked' framing — the existing API is usable, just different from original expectation.