I have bought fish, inverts, and plants using priority and express/overnight mailing. In all cases everything survived except for one incident which was due to carriers neglect. What some carriers do is they will not leave the package or will decide to be a jerk and not show up at your door and hold the package all day on their truck and lie and say they showed up and you were not home.
This happened to me twice. One incident the fish died, it was the summer and the drive never came to my door, instead he decided to take the fish on a ride all day in his truck. I went to the post office at 5:30 to get the package and the fish was boiled. The other time the UPS driver never came to the door and lied so I had to go to their facility and pick it up, fish was ok though. The thing that pissed me off the most about it was it said LIVE FISH right on the package and the carrier did not care.
But back to shipping. Priority mail is usually fine for a lot of things. Overnight/express is only necessary for certain types of fish etc. Fish handle it pretty well and should arrive no problems. The only problem like I say, is mail carrier neglect, but it doesn't happen often. I have had stuff shipped to me, and I have shipped plants, fish, and inverts, and everything worked out right, except for those 2 incidents.
And the reason people don't guarantee on priority is cause it is not a guaranteed service. They say it is a 2-3 day service, but anything can happen. If there is a screw up with the shipping company and the package gets lost or delayed, and it CAN happen, there's nothing you can do. But for the most part it doesn't happen often. Express/overnight is guaranteed by shipping companies.
It is really your call. Most fish, inverts, and plants ship fine priority. But if these are rare and expensive, or just mean a lot to you, then the overnight/express would be a better choice if it makes you feel better.
