1. Gather proper data for the dummy cards, including visa, master, etc.
2. Gather payment gateway information, like PayPal.
3. Gather error codes related to the payment gateway.
4. Know the parameters that are passed through the payment gateway.
5. Know the information that are passed through query string.
6. Check the language of the payment gateway and the application.

References – https://www.360logica.com/blog/

1. Change the language of the payment gateway during the payment process.
2. Check whether the important components are retrieved or not after the payment.
3. Check the response if the payment gateway stops responding.
4. Check what happens in the back end and if the session ends.
5. Check what happens if the payment fails.
6. Check if the database stores the card details or not.
7. Check the errors during the payment process.
8. Check the response after turning the pop up blocker on and off.
9. Check the buffer page between the payment gateway and application.
10. Check what happens after successful payment.
11. Verify the transaction process.
12. Check the messages and verify it after successful payment process.
13. Check the currency format of the payment.
14. Check if the payment option opens the respective payment gateway after selection.
15. Verify the default debit card option.

References – https://www.360logica.com/blog/