PetExec only allows you to select one type of payment at checkout, but you can easily take more than one payment.
1. Enter the first payment type at the checkout screen.
2. At the bottom of the receipt, click on the "Click to Modify this Order" link to immediately modify the order to apply a payment.
3. In the Modify Order screen, select the second payment type and enter any additional information and the amount applied. Press the "Process Order" button to add the second payment type.
You can verify the multiple payments are there by scrolling to the "Payment History" section of the Modify Order screen. There will now be multiple line items showing both payment types.