i try to use the priority mail flat rate boxes as much as i can. but when you start getting into the bigger items, shipping can really jump. using the box from hd, a chopped tourpak would run about $75.00 to ship. for those items that you can't get flat rate boxes for, you can go to usps.com and work up shipping estimates there. i recommend getting an account. if you pay for your postage online, you can save a few bucks. i just shipped a box to xtreme dimension painting, it would have run $43 if i paid at the po, but purchasing postage online it was $37.
you can also order a bunch of supplies from usps.com as well. i got a handful of the customs envelopes. i buy my postage, download the label (its easier if your printer takes a poop to reprint a pdf), print it on 8.5x11 paper and use the customs envelope to stick it to the outside of the box. supplies are free, save a few bucks on postage, win-win
