here's my tips: pull the tubes, pack them properly, and take pictures of all that. they're the most likely to fail and you can prove due diligence to the buyer, the shipper, and whatever middleman there is.
aside from that, UPS is fine especially if they do the packing of the amp. Just make sure you insure the shipping for the total amount of whatever refund you'd need to provide should anything happen.
I followed those two rules and still had a rectifier tube (6x4 are notoriously fragile) go bad in shipping. when the buyer complained I clearly laid out their options to surrender the amp to UPS for destruction and claim the insurance money or buy a $20 rectifier tube and move on. They kept the amp, I kept the money.
