We sent the USPS Priority Mail (2-3 days) box for 8.95, you stuff into it whatever will fit. We sent ahead our twin Coleman mattress (we have a 12-yr-old DD and 10-yr-old DS who can't sleep in the double pullout-they're practically my size) and some toiletries. When we checked in at BWV, the CM told us we had a package waiting for us. Normally, we just pack the mattress, but we were looking for alternatives to lugging it along.
Owners Locker initially sounds like a good idea, BUT, it's more money than we're willing to spend, considering we only travel carry-on anyway.
FYI, we gave our package to the front desk when we checked out, and didn't see it for three weeks after we returned home because it had to go ground.