You have to watch out for those priority boxes. The Postal workers get really testy if you use them "in the wrong" manner. Of course, the rules that govern their use are clear as mud.
One time I was mailing a quarter (I think it was only one, but it might have been two-not many at any rate) to our Dawn. The postal worker felt the coin in the card/envelope and asked. I told her what it was. She told me I'd have to send it registered mail and it would be some ung-dly cost like $22 or something. I took it back, went to another Post Office, kept my mouth shut and mailed it for a sensible price.
They are taught to increase/protect the revenue. That means, get you to purchase the most expensive form of service they can.
Thank heaven for online info. We can now be somewhat informed, eh?