If you miss it (package not there yet, or post office closed when you come through), leave a signed, written request at the post office for them to forward the package to West of Nowhere, KY, 32109.
For what it is worth... You can make these arrangements to forward them at any post office, not just the one the item was mailed to. Personally I'd just buy the maps all up front unless your plans are likely to change.
True, but if you do it from a different post office you may run into a stickler who'll route it through someplace in Missouri (St. Louis, maybe?). It can take a week to ten days before the receiving post office will get the notice and forward the package. You can drop a sheet of paper (even without a stamp!) into the mail drop at the post office to which the package was originally addressed, and they'll forward it the next day they're open (if it was closed when you came through) or as soon as the package arrives.