If your client is in your office or has provided the credit card information to you, you can use that information to make a payment to an invoice.  To do this go to the invoice, (or create the invoice), in your client's project, click on the Preview button, then click to pay the invoice:

When you click pay invoice you can add the information from the card and then click to pay and it will process the payment.

KeyWords: payments, pay, invoices, credit card, debit card, 

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