What amp and guitar are you looking at specifically? Amps I wouldn't recommend buying a random one off any US site simply because of the voltage difference. If you go directly to the builder or through a dealer and request for a local voltage, that's another story. The safest method would also be the more pricey method so you've gotta think if you're willing to pay the extra premium for better shipping options such as FedEx and if you want an expedited shipping.
I've shipped guitars via FedEx, DHL and even USPS was okay. Generally though, if the guitar exceeds say 5k USD, I'll just pay the difference for FedEx priority.