WayForPay integration in Shopify — HMAC signature & auto-submit

Task: connect WayForPay to a Shopify store with automatic
order amount passing and redirect to the payment page.

WayForPay specifics:
— Requires HMAC-MD5 signature (vs SHA1 used by Fondy)
— Payment via HTML POST form (not API redirect)
— Amount in base currency units (not cents)

Solution:
— Vercel serverless generates a signed HTML form on the fly
— JS auto-submits the form immediately on page load
— EUR/UAH support depending on store currency
— Currency format validation to prevent API errors

#shopify #wayforpay #payment #vercel #nodejs #hmac