Which one is the best to use? USPS, FedEx, UPS?
FedEx and UPS charge the buyer brokerage fees. Do not use them.
I have been very happy with USPS International Flat Rate envelopes and boxes.
Just don't use the small flat rate box or envelope. They don't provide tracking. Otherwise USPS Priority Mail International is just fine.
USPS is pretty good. Gives you a couple of options for cost/time