Everything you need to know about payment gateway testing

When you’re talking to developers, it helps to have a framework. We break payment gateway testing down into four distinct pillars. This ensures that not only does the pay button work, but that the entire infrastructure behind it is rock-solid.

Functional tests

​​Functional testing makes sure that the application behaves as expected. This could include placing orders, confirming correct calculations for multiple items, applying codes, and estimating taxes.

Functional test cases for payment gateways

• Testing payment gateway success: Does a valid card number direct the customer to a thank-you or order-confirmed page? Does the system correctly handle different card types (Visa, Mastercard, and American Express)?

• Calculations & logic: Are the shipping costs and VAT/taxes calculated accurately based on the user’s address? If a customer applies a discount code, does the gateway reflect the new, lower price?

• Error handling (Also known as ‘negative testing): What happens if a user leaves the CVV field blank or enters an expired date? The system should provide a helpful, human error message rather than a technical code.

• Successful transaction: Verify that the payment was processed with valid details, that funds were transferred, and that confirmation was received.

• Failed transactions: Test with invalid card numbers, expired cards, incorrect CVV, or insufficient funds.

• Declined or blocked cards: Ensure the system properly handles blocked or fraud-flagged cards.

• Network or connection failure: Simulate interruptions to ensure the system doesn’t double-charge and can handle transaction timeouts.

• Partial payment or session timeouts: Test behaviour when a user abandons the payment page halfway, or the session expires.

• Currency conversion: Validate the conversion to the user's location's currency.

• Refund management: Check that refunds (full and partial) are processed and credited back correctly.

Integration tests

On a basic level, your website is a collection of different apps talking to each other. Integration testing ensures that your website, payment gateway, and backend (e.g., Shopify, Magento, or an ERP) all speak the same language.

If you are using one of our pre-built integrations, much of this logic is already optimised, but testing remains a vital step to ensure your specific configuration works perfectly.

Test cases for payment gateway integration

• Order creation: Once a payment is authorised, does an order appear in your store’s dashboard with a ‘paid’ status?

• Webhook handling: Does your site receive the success ping from the gateway to automatically trigger a confirmation email to the customer?

• Refund & void scenarios: Can you initiate a refund from your website’s admin panel and ensure the transaction status is updated correctly in the payment gateway?

• Multiple payment methods: Test credit cards, debit cards, net banking, and UPI.

• Return URL and redirection: Are the customers correctly redirected back to your webshop after successful or failed purchases?

Security tests

This is the most critical pillar for building trust. Security tests make sure that sensitive data (such as card numbers) is never exposed to hackers and that you meet legal requirements. At Mollie, we prioritise this through our fraud and risk management solution, which helps you maintain high security without adding friction to the customer journey.

Payment gateway security test cases

• Data encryption: Is the connection between your site and the gateway protected by SSL (the little padlock in the URL)?

• PCI DSS compliance: Ensure customer card data is tokenised, meaning the actual card number is replaced with a secure code so it’s never stored on your servers.

• Fraud prevention: Test what happens when a transaction is flagged as high risk. Does your system hold the order for review?