[pct-l] Another bounce box question

[res46 at cornell.edu (Rebecca Emily Selling)]

How exactly does the bounce box idea work within the US mail system?  I
read at some point that if a box is sent priority mail it can be forwarded
for free.  Is this true?  Is it just one free forward or multiple?  Also,
a small logistical question is, how does one acquire tape to seal boxes at
the post office?  I come from Philadelphia where tape is not supplied at
the P.O. and it seems silly to keep buying tape at each resupply stop. 
Thanks a million for all answers!