How do I charge for additional cats?

Hi community!

I have been cat sitting for a client for about 2 years now. The owner has 6 cuddly cats and they are fun to take care of. But I've I feel I've been undercharging the client for each sitting. My rate has been $50 for cat sitting, but I recently starting asking for $70 (my standard rate) because of the additional cats. If I try to modify a booking to say additional cat, it switches the fee to just $25.

How can I accurately modify this type of booking to reflect what I should actually be charging?

Any ideas or advice would be helpful!

Thanks,

Caleb