I am being charged a "cost adjustment" when I went to book my dog sitter. I've never been charged this before. What is it and why am I being charged?

The best thing for you to do is ask your sitter, who would have been the one to make the adjustment on your bill. It could be that the length of the stay exceeds the normal 24-hour increments, but that would be just guessing on my part. Ask your sitter.

If your sitter can't explain (which I would find strange), then contact Rover Customer Support at their phone number [Edit: Rover’s contact options have changed. Visit the Rover Help Center at https://support.rover.com/ to find the phone number, help articles, or chat with the team]. This isn't the place to contact them.