For me so far it has depended on how much they buy.. flat rate envelopes are usually my best bet (they cost 4.80 plus .65 if you want delivery confirmation) but if they're only ordering 1-2 bars sometimes it's too much for them.. if they order a LOT, or heavy stuff like body wash, a flat rate box is best for me (9.80 plus .65).
Those prices are assuming you purchase at the PO - they're slightly less and DC is free or cheaper if you purchase through paypal or USPS click n ship.
maybe so.. we don't have anywhere to ship DHL around here, we have to drive 1.5h to find one, which makes it no longer more affordable for us. I didn't even think to recommend them since we can't use them.. but yes, definitely shop around
I work for USPS, do you get home delivery or store delivery? If so the carrier will pick up anything that you have to ship, just go to usps.com and do click and ship, then schedule a carrier pick up. I have these on my route all the time.