You would first need to have enabled the receipt of payments via credit cards. If you have already done so, here are the steps to pass credit card processing fees to your customers:

Step 1: Log into Vault

Step 2: From the side menu bar, select "Invoices"

Step 3: Select the "Customers" tab

Step 4: Select the specific customer card

Step 5: Enable "Credit or debit card"

Step 6: Enable "Pass processing fees to customer"