Many sitters offer discounted rates for a longer stay. It sounds like you've booked with someone already, in which case you need to discuss with that sitter whether or not they could offer a discount. They can contact Rover to refund a portion of your total without canceling the stay. However, depending on the sitter, they may not offer a discount (or you've already paid the discounted total, since Rover automatically uses their extended stay rate when you request a booking for a date range long enough to fall into that sitter's extended stay parameters. If you haven't yet booked with someone, be sure to discuss rates and discounts before finalizing the booking.
Pet care is a significant cost of travel, much like booking a hotel for yourself, except that pet sitters are generally much cheaper than a hotel and provide way more for your pets than just a bed to sleep in and a continental breakfast. A lot of pet owners assume that because sitters are dog lovers they should jump at the chance to watch their baby for little or nothing, but it's a lot more like child care. You'd never expect a daycare center to just want to watch your child all the time, no matter how wonderful they are. You can get a cheaper rate using a friend or family member, or the high schooler down the street, but if you want professional care from experienced providers, you're going to pay quite a bit more for all the benefits that includes.