I know plenty of folks who have either shipped or received shrimp completely cross-country (California to East Coat) via Priority Mail and never had a single problem with the shrimps arriving alive and healthy. As long as the shrimp are healthy to begin with, and packed properly, there is really no need for Express shipping unless there are really unusual circumstances.
I know for myself, when it comes to buying shrimp online I am always very wary of any seller who insists that you "must" choose Express shipping. That's hogwash. Typically in that scenario one of three things is happening:
They are charging you significantly more than their actual shipping expenses (e.g. charging you $45 for Express shipping when it is only costing them around $30 to ship the parcel)...in other words, essentially taking advantage of the cost of Express Mail in order to over-inflate their total selling price and thus pocket even more money for themselves.
They are raising inferior quality shrimp, thus a 3 or 4 day trip that healthy shrimp would survive without problems, this particular seller's inferior quality shrimp can't survive...thus the seller insists on Express Mail since they've learned from past experience that their shrimp don't survive when shipped via Priority Mail. (Note this could also be the case if a seller tries to ship adult shrimp--which typically fare much worse in shipping than juvenile shrimp do.)
The seller doesn't know how to package their shrimp properly, and thus they have experienced shrimp losses via Priority Mail in the past where, if the shrimp had been packaged correctly, the shrimp would have done fine. Do they include something (small) in the bag for the shrimplets to hold on to? Do they use Kordon breather bags? Do they use hot/cold packs if outside temps call for it? etc.
The moral of the story is: healthy, juvenile shrimp, well-packaged, should not have a problem being shipped even completely cross-country via Priority Mail. That means ~ 3 days, and the shrimp should be fine.
Lets put it this way... I'm in Cincinnati, and I ship 10-15 boxes of shrimp a week (on average). I prefer to ship Priority on SATURDAYS, as they seem to travel faster (yes they do travel on Sundays, just not delivered).
Only twice have I had buyers want Express and that was because they wanted their shrimp fast, and they were going to be leaving to go somewhere and didn't want to miss them.
Last week I shipped a box from Cincinnati, to Dutch Harbor Alaska... Priority. I shipped out on a Saturday and by Friday I had positive feedback. Although I am not sure which day they were received my guess is that it was probably Tues or Wed... either way, the Snowballs traveled well and I was given good feedback.
Earlier this summer I shipped a box Priority to a US Navy personnel from here to Massachusetts. After I had made the shipping label he called and informed that there was a mistake on the shipping information and he gave me his old address. I went to the Post Office and the guy at the counter told me to cross out the information and write the correct address... and not to worry about the barcode on the box; that it meant almost nothing. Wary, but running later for work, I left it there. A week later and I was given word that they had not shown up. Great... Oh yeah, these were (12) 1.5" fish (Cichlids - Julidochromis species) that were being shipped in a double breather bag.
Turns out was that the barcode was read, and it was shipped to the next PO. It said the box was at the right place, but someone forwarded to the next post office as that was what was written on the label. When the 2nd post office received it, the barcode was read again, it went back to 1st PO. At the Post Office they call it a "looping package"... after 3 tries it gets kicked out (usually) and it's investigated. After 2 1/2 weeks I get a phone call from my customer saying he finally received the box... and oh yeah, out of the 12 fish, only 2 were dead!!!!!
Moral of the story: The breather bags are an amazing "new" technology to this hobby... and as long as the water is clean, the fish are healthy, and temperatures are within a tolerable range, they will do well.
2-3 days should be good.
Sometimes if you get from another country or wrong shipping can take 4-5 days... They still would be fine long as the sender packed them well.
Last shipping I had was about 6 days and there were no death.
I generally prefer express shipping on crystal shrimp and blue tigers and other more sensitive (and more expensive) shrimp. They would probably be fine with priority but express is more reliable and it shows up much earlier to my house 11am vs 4-5pm with priority (important to minimize time riding around in a 110F+ truck). Also if the seller is guaranteeing live arrival I can see them requiring express on more expensive species to limit their risk.
I agree with the answers above, but one thing not mentioned is that every so ofter with Priority Mail a package will get misplaced and not show for up to 5 days or more. This happens more often than you think and I have been using Priority Mail in my other business for 10 years so I speak from experience. This does not occur with Express Mail.
I tend to let the customer decide with shrimp, but encourage Express with rarer more delicate shrimp. Winter shipments out of the Southwest can get very cold VERY FAST when it is below zero outside and express is always best in my opinion under these conditions.
Percentage wise? 80% of my shrimp go out Priority Mail and I rarely ever see a DOA. I do however worry some over shipments to the East Coast from here in AZ, but so far so good.