Based on my experiences, you wouldn't want to turn in the receipt from hotel because it will probably reflect a rate lower than what you paid Expedia.

Expedia and Priceline don't only make money on thier service charge, they have the potential to make additional money on the difference between what the hotel sells them the room for, and what you pay them.

The few times I have been able to get receipts, there has been a few dollar difference between what I paid Priceline and what the hotel shows. It's been as low as $3 and a high as $10.

I've never begrudged them this difference, the rates I get have always been cheap. It's more been a matter of curiuosity. But it is hard to get a copy - I've been told it violates the agreement between the hotel and these companies to disclose the actual rate to the guest.