I just received an amp yesterday in a wooden crate from UPS. If it had been in a cardboard box the amp would have been destroyed because the crate is very damaged, one hinge is torn off, both clasps destroyed that hold the lid down, wood pieces missing and other wood parts with large, wide cracks. It obviously took a nice fall at least a couple of times.
If you're capable and are concerned putting a wood box around the outside and putting feet on your mini pallet will help alot. That's what I did last time I shipped large speakers(Magnepan 3.5s) and it worked well.
Uhaul sells some nice double thick boxes. It may not hurt to double box the amp. I can tell you another story about an amp that was double boxed. The outer box looked like someone rolled it down a rock cliff and the amp surely would have been destroyed if it didn't have that outer box. It was a Parasound HCA 3500 with the factory double boxes. That had been Fedex that time. No guarantees although it seems like USPS has a lower damage rate but I've used them less also.