@Lorna170,
Whenever I have to send reminders to guests to provide more information or payment, I state it in a manner that they realize their inaction will impact their stay. I usually say something like,
"Hello XX,
We are really looking forward to your stay at our cabin next month. Since you plan to bring your two pets with you, this is a reminder to pay for the $75 pet fee. If you have difficulty finding the payment request message, you can find it by going to www.airbnb.com/resolutions. Your prompt attention to this matter will help to avoid any delays with sending you the door code for your check-in.
Please feel free to contact me if you have any questions."