To send your QSL cards through the bureau, arrange the cards alphabetically by call sign. Stateside cards are free. All cards outside the U.S. postal districts are 10 cents each. If you change your call sign, keep the bureau informed. They will also mark each envelope you send them so you will know how many envelopes are left.
All mailings are at the end of the month.
If you want your envelope sent to you only when there are so many cards in the envelope, immediately below your call sign in the upper left corner, indicate how many cards you desire before mailing to you. Also, when you get a callsign upgrade you will have to keep envelopes with both call signs on file at the bureau in case some late cards come in with the old call. That's it easy as can be. A free service that can save you lots of money in stamps. If you do not send an envelope for your QSL cards that they have, they will be destroyed after 6 months.