I have clients who want me to sit their dogs for months at a time and they find it easier to have me go buy their dogs food. If I put the cost of food under adjusted costs will Rover still take 15% of that?
As Laura R. stated, Rover will take 15% of whatever is charged through the site. You could offset this by increasing the rate you charge and explaining to the client so they are aware of the 15% cut for Rover. I've had some clients not know about this cut, and it helps put into perspective some of the higher end rates being charged.
Also, while Rover specifically forbids paying via cash for stays, clients are allowed within the terms to tip their sitters. Perhaps there is a way your clients could pay for the stay via Rover and as a tip, reimburse you for the dog food. This way Rover won't be sort of offhandedly turning a profit from dog food sales -- something I think they would agree with.
They get 15% of whatever the total after adding in cost adjustments, regardless of what the final total ends up being.