"In Bulgaria they would not recognize stamps printed more than 2-3 years ago,"

"1.2 Postage Stamps Valid for Use

All postage stamps issued by the United States since 1860, unless listed in 1.3, are valid for postage from any point in the United States or from any other place where U.S. Mail service operates. Precanceled stamps may be used to pay regular postage and fees for extra services if the mailpiece is endorsed under the standards for the class of mail and service requested. Precanceled postage may be used only by permit holders authorized under 3.0. Unless excepted by standard, the total postage affixed must equal at least the postage charge for the class of the mail and, if applicable, the fee for the extra service requested. All nondenominated postage and makeup price stamps, including official mail stamps, are valid at the original prices of issue. Exception: Semipostal stamps.

1.3 Postage Stamps Invalid for Use

The following are not valid to pay postage for U.S. domestic or U.S.-originated international mail:

Postage due, special delivery, special handling, and Certified Mail stamps.
Stamps of other countries.
United Nations stamps, unless on mail deposited at the United Nations, NY.
U.S. stamps that are mutilated or defaced; cut from stamped envelopes, aerogrammes, or stamped cards; covered or coated in such a manner that canceling or defacing marks cannot be printed onto the stamps; or overprinted with an unauthorized design, message, or other marking.
Nonpostage stamps, such as migratory-bird hunting and conservation stamps, U.S. saving and thrift stamps.
"