1. Open Apps and Integrations and choose the Payments category. Select the required gateway and click Activate.
2. Follow the documentation for that exact module to obtain the correct credentials. Keep live and test accounts clearly separated.
3. Enter the required values, a customer-friendly display name and the intended order-form visibility.
4. Review currency support and any Convert To For Processing setting. Confirm what amount and currency the customer will actually authorise.
5. Save the configuration and complete any provider-side callback, webhook or account-verification setup.
6. Run a controlled successful payment and a failure case using the provider's supported test process. Confirm the WHMCS transaction, invoice balance and expected automation.

Determine whether the module uses a saved payment token, a provider subscription or a manual payment on each invoice. Explain that behavior in customer-facing payment instructions. Do not assume a payment made once automatically establishes future authorised collection.

Review product-group gateway restrictions with a mixed cart. At least one suitable payment method must remain available for all products in the cart. Test invoices generated in each offered currency as well as the initial checkout.

Before switching from test credentials, record the approved merchant account, callback destination and expected transaction description. Restrict access to gateway settings and retain the credentials in the approved secret store. Perform a controlled live verification under the company's payment policy, then reconcile the provider result.

Keep an existing gateway available while its subscriptions or stored tokens still require it unless the migration procedure explicitly transfers them. Disabling a checkout option and retiring an integration completely are different changes.

• Troubleshoot WHMCS payment callbacks and unpaid invoices
• Configure supported saved payment methods in WHMCS
• Add and maintain currencies in WHMCS
• Run an end-to-end WHMCS test order before launch

• WHMCS: configure a payment gateway
• WHMCS payment gateways
• WHMCS product groups
• WHMCS pay methods