I am not sure I have said this on this thread before, but it bears repeating.
1) You will almost never be asked for ID.
2) Even if you are asked for ID, they will almost never complain about whatever you provide. I provide a letter that basically says, "please give this contractor the government rate," and it almost always suffices. The clerk almost always just wants to check the box of, "I asked."
3) Even if they complain, they will almost always back down if you say, "Look, this is what they gave me. You can call my office in the morning if you have a problem with it."

As long as you are actually entitled to the government rate and have some piece of paper or ID, I would not worry about it.