Keeping beef jerky dry?
There are several options:


wrap jerky in paper napkins/towels before putting in the baggie
put jerky in a paper bag and then in a plastic one
put some uncooked rice, as Optionparty has mentioned


The method you choose also depends on how long it takes the mail to be delivered. Rice is the most long-lasting way, but it will also add the most weight to the package. Paper bags and towels are equally effective, but they won't do for longer than a few days.

Whichever you do, make sure the baggie is well closed to prevent any moisture from outside getting in.