Originally Posted by Bobster

Generally, to qualify for tax exemption, the payment must come directly from the government to the hotel. A traveler who uses his personal credit card to pay with later reimbursement does not get a tax exemption. You need a government credit card, in which case the card itself is all the proof you need, and possibly forms to be filled out. And note that DC has a law prohibiting photocopies of the credit card for security reasons.