I've received several manuscript submissions lately that were sent to me Priority Mail. Just opened one today--that has been sitting on my desk for a month.
Folks, Priority Mail gives you no advantage in the query/submission process--especially if you're sending unsolicited manuscripts. Send it Parcel Post. Or Media Mail. Or even First Class. All are usually quite a bit cheaper than Priority. Unless an editor specifically requests that you send your manuscript Priority, save your money for more useful stuff--like toner and paper.
And if you really want to save money, do not send a full manuscript as your first contact with a publisher. Send a query letter. And don't let anyone tell you that it's harder for a publisher to reject a full manuscript than it is for them to reject a query or that we're more willing to read a manuscript sitting on our desk than we are to ask for one to be sent. That's nonsense.
When I see an unsolicited manuscript show up on my desk marked Priority Mail, I think, "Poor soul. They don't have a clue how this business works. This manuscript better be good because I'm going to have to spend extra time educating this author." When I see a well written query letter show up on my desk, I think, "Great! They've done their research. They know something about this business. I probably won't have to hold their hand all day, every day...yes, I'll give their manuscript a chance." (Assuming, of course, the query is for something that I'm looking for.)