As mentioned, purchase the packing materials so you can strap the speakers, amps, or whatever to a pallet yourself. Find a business that accumulates excess pallets and will give you one, or simply buy a pallet.
If you don't have access to a fork lift and cannot find a shipper to pick up your loaded pallet from your home using a lift gate truck, then assemble the entire thing on the back of a pick-up truck, shrink wrap and strap the boxes securely to the pallet, and drive it directly to the shipper that should have a forklift to unload it from your pick-up truck into their warehouse (of course, verify ahead of time they will unload it for you).
As you pointed out, the buy-to-try program is more consequential for large speakers and heavy amps than for lighter, one-box gear such as DACs and preamps, due to the cost and time commitment necessary when you decide to sell and ship them. Also consider, even though some of those large and heavy items can be shipped in their original boxes using a regular carrier, if you don't palletize them and ship by freight the risk of damage goes up. If you really want pain, try negotiating shipping damage on expensive speakers or amplifiers with a buyer and shipper.