Industrial....you say flat rate USPS boxes. Is that what you really mean? Because if so, flat rate doesn't move exactly the same as priority USPS boxes (like the 7x7x6 which is commonly used). And the only flat rate boxes I've seen are not very deep, not deep enough to put a fish shipping bag into it.
If you shipped priority and it took 5 1/2 days, that's an exception rather than the norm. I've shipped all over the country and I think there's only one person who happens to live in nowhereville, NC, where it takes an extra day or two for packages to get to
If you're going to ship fish or plants or whatever from time to time, you need to ask your post office what time they send their trucks to the next postal location. Here where I live it is rural. One truck a day goes from my post office to the next sorting facility which is an hour away. I also happen to work across from that next sorting facility and it's in a fairly large area, much more volume mail, so their trucks leave at 1:00 and 5:00 before the post office closes. Now stuff still moves through this sort facility after 5:00 so if I ship from my home town, it's not going to get to the sorting facility until 6 and probably get processed somewhere between 6 and 10PM. Then on to the next sorting facility which for me is Kearny, NJ, then from there on to the plane. You may have taken your package to the PO after their daily truck left, which means it sat overnight not going anywhere until whatever time the next day their regular truck leaves. All my california packages get there in 3 days from NJ.
