We charged by the cart as well, back in the hay day we would get .50 a cart. That was for disinfecting and steam cleaning each one. We never move carts when we do S/C we tell them when we arrive that we will be P/W the center and if they wanted it cleaned under the carts they should move them off the sidewalks.
Most times your going to be talking to an assistant mgr at night, and alot of the times they will not want to help you. I have found out that if you tell them, if they refuse to move them then I need to get your name so when the Store Manager comes in and asks why its dirty under the carts I can tell him that you refused to move them. 99% of the time they will be out there moving carts.
If you are contracted to clean them then I wouldnt have any problem moving them out because you have to handle each cart anyway. You can use a very light housewash mix and steam to rinse, you may want to ask if they need their bakery racks done as well.
