How do I charge for special food I had to but for an extended stay?

You will want to do the reimbursement off site since the service fee will be taken out if you do it through a booking. You should send the client a message through the Rover messages with a picture of the receipt and letting them know the total you need to be reimbursed for and maybe let them know the payment method they can take care of it with. That way you have a record of it through the Rover messages but you don't have to worry about losing out on part of the reimbursement to fees.