Well, I tried! When I first started out (I'm still pretty new to hosting) I learned quickly that guests are quick to accept refunds, and slow to accept "extra" charges via the resolution center or the "change reservation" feature. Yes, most of the time they eventually pay, but not without reminders and prodding and even threats of not being able to check-in if the pet fee isn't paid.
So, I added the pet fee into my cleaning fee and gave a REFUND to guests who did not bring a dog. This worked beautifully, although I wondered if it made my listing seem too expensive to those who didn't pay attention.
So I switched back. And guests never fail to disappoint me! The first guest with a dog after the switch ignored the pet fee altogether even though it was clearly documented in my rules and via our messages. Still trying to get payment for that one. Second guest still has not paid the pet fee even though we have exchanged messages since the request was sent. They will NOT be allowed to check-in until it is paid.
So......I'm going back to including the pet fee in the cleaning fee and offering refunds. When I offer a refund, the guest accepts within MINUTES!
Airbnb, IT'S BEEN ASKED FOR FOR YEARS SO ADD A PET FEE OPTION ALREADY....PLEASE!