For such questions it's better to negotiate that before booking and then make and accept a special offer with the higher price and have everything written in the message thread. If you already booked, your host can send you a request to alter the booking to the higher price (including the 25$) and you accept that offer. After that the 25 are taken by airbnb from your card and paid to the host with the rest.
I see no reason to pay that in advance by paybal - the pup will not arrive before you?
If it's an independant service, like a refund for a fee, and you trust each other, you can also pay it at arrival in cash. (You did not indicate for what the money is due). Sometimes I buy telephone cards for my guests, leave them on the table for them or lend them a phone with the card already installed and they leave me the money when they leave. Some people ask to have a thing at arrival, so I buy it and leave it with the ticket and they leave the money.
The terms should protect you against extortion, that there be no fees as a surprise sprung on you. If you ask for something special and the host provides it, you can refund him in a way which suits you both. If it's part of the rent, pay it via airbnb.
