Complaint Review: Best Buy

I bought a Hewlett Packard computer package from Best Buy which included a $100 rebate. I jumped through all their "hoops" in filing for my rebate. I mailed it in on Dec. 4, 1999 and their rebate said all requests must be postmarked by 12/6/99 so my postmarked request was under the dead line. On Jan. 24, 2000 I got my rebate request back (unopened) with a stamped message on the envelope stating that the offer had expired. I sent the whole thing back to Best Buy. P.O. Box 6985, Young America, MN 55558 along with a letter telling them that I had complied with all their request, including getting the request "in on time". I got that letter back (unopened) with the same message "offer expired" on the envelope.
It hardly pays to hire a lawyer to go after them on this so what else can I do? I have kept all my papers as evidence.
Sincerly, Gary L Randgaard