The big benefit of shipping by Registered Mail comes when you hit expensive guns. There is a whole different schedule for insurance fees for registered. It is very cheap. the savings in inusrance fees will more than pay for any difference in other shipping fees. I dont have the registered chart in front of me but I have shipped some 5 figure guns and used them.
If time is of the essence, then Priority Mail by all means.
read the fine print with UPS and FEDEX. They will not pay the claim on an expensive gun no matter what you paid for insurance or that they accepted it for carriage. I have been down that road. They have a sub classification for guns. They will take your money for insurance but try to collect when they snap that buttstock of that Purdey SLE in half.