✓ Create gateway class with required methods ✓ Register gateway on plugin initialization hook ✓ Implement payment processing method ✓ Implement webhook handler with signature verification ✓ Implement settings form for API credentials ✓ Store and retrieve gateway settings securely ✓ Update invoice payment ledger on successful payment ✓ Update invoice status (paid/partial) ✓ Handle payment errors and edge cases ✓ Test with sandbox credentials first ✓ Verify webhook delivery and processing ✓ Document settings and configuration for users