What if a pet owner wants to pay me in cash?

I am new to http://Rover.com and was a little confused on how the payment/invoice process worked. I accidentally told a client the wrong amount for the total charge before we booked it but she was so nice, I was okay with leaving it on the cheaper end for her. I had a meet and greet with her today and from the way she spoke about paying me, it sounded like she wanted to pay me in cash because she said she was going to leave me more than on the invoice. I also prefer to be paid in cash, if the client is comfortable with that. Is that possible? I don't want the client to leave me cash and then her card get charged once the stay is completed; I would be so embarrassed and feel absolutely awful.