AirBnB is pretty bad about special pricing. We have a surcharge for 1-night stays, but the only way to do that is this (would work the same for pet fee):
1) Put a note such as "*** If you bring a pet, there is a non-refundable $50 additional fee ***" at the top of your Description to put guests on notice.
2) When a guest messages, confirm with them that they accept the $50 fee.
3) If they are inquiring and haven't booked yet, extend them a special rate that includes the $50 (e.g. if they book for 3 nights at $90 per night, extend a special rate of 3 x 90 + 50 = $320.
4) If they've already booked, you need to cancel the booking, then extend them a special rate. When cancelling, write a note that says that you're only cancelling to change the rate, and will send them a new rate for approval in a few minutes, so they don't panic and think you're cancelling on them.
IMHO this is way cumbersome, but only way I've found to do it.