USPS, if you don't tell them WHAT you're shipping. a LOT of bubble wrap, a LOT of duct tape, and some newspaper (not only for padding, but also sound insulation, to muffle the "sloshing" sounds). at that point, size versus weight, it is usually worth it to weigh and measure your parcel versus flat rate. AND this can all be done from your home computer, so you don't get nosy know-it-alls at the post office, blah blah "you can't ship that" blah "laws" blah "i need to get my manager"...
i have shipped a couple few here and there, and only had one VERY minor issue, FWIW.
EDIT: eddie, you damn ninja!