Correction:

What I said earlier about government credit cards was incomplete and misleading.

Federal cards may be billed centrally or billed to the individual. Only the centrally billed cards are tax exempt. There are digits in the credit card number that indicate they are tax exempt, therefore the credit card receipt with the number should be only proof required for audits. The treatment of individually billed cards is determined by local law.

These links explain it better:

http://otr.cfo.dc.gov/otr/lib/otr/sa...entpackage.pdf

and

State Tax Exemption Forms Overview