Is it possible for me to manually enter my client's credit card information?

Yes, you can collect and enter your client's credit card information yourself.

To do so, navigate to the Client tab, select the client for which you'd like to add the information, and scroll down to the very bottom of the page to the "Payment Cards" section. Click on the "Add Payment Card" button.

Then, enter the credit card number, expiration month, year, CVV number, and the cardholder's name.