Bill Conners writes: "By mistake, I bought a $1 postage stamp from the miniature post office in the 1300 block of F Street.

"I took it to the Irving Street station and asked if I could use it, along with $2 in cash, to buy a $3 book of stamps. The lady told me that the post office accepts no exchange of stamps. Is this correct?"

Yes and no. A spokesman for USPS explains that, as a general rule, you cannot exchange stamps for different stamps. On the other hand, the Postal Service realizes that people sometimes make mistakes and buy stamps they can't use. When such a person explains within 48 hours that he has made a mistake, USPS will permit him to trade in his purchase for stamps of another denomination.

The USPS spokesman told me that Conners should make a second attempt to trade in that $1 stamp, and this time he should be sure to explain that he bought it by mistake.If the clerk does not cooperate, Conners should talk to the supervisor or call the USPC Consumer Advocate's office (245-4514).

There's another alternative. He can just hold on to that $1 stamp until the price of mailing a first-class letter reaches $1. It won't take long.