My son is having his birthday party at the movies this year, which requires an explanation to invited guests that although we know an approximate window for the movie start time, we won't be able to communicate the exact time until the theatre provides it the week before the party.
Rather than trying to cram all of this info onto the single line provided in most standard invitations, (and getting hand cramps from writing it 12 times), I got Avery shipping labels (#5164 which is 3.3" x 4"). I typed the details once, printed 2 sheets of the labels. Peel, stick, and done!
Even if you've got just the basics to provide, this would save a lot of repetitive writing.
Along these lines, I've used mailing labels to create recipe directions for food gifts (specifically, the homemade chai concentrate we gave my kids' teachers last year — just stuck a label onto each bottle).
But back to the birthday party invitations: another, possibly cheaper option would be to make your own simple invitation using a word processing program and a few bits of clip art. Print as many copies as you need (or print and photocopy onto colored paper), fold, staple, and mail.
Do you mail birthday party invitations? Or do you use a Web-based invitation service such as Evite? Are there Evite alternatives you prefer?