Gale...you are correct. Be careful about turning the post office boxes inside out. The post office has the option of opening a box if they think you are using one of their boxes for other purposes...you are only supposed to use what it is provided for.
I use Priority Mail all the time. Spend the money and get yourself a scale for weiging your packages. All of my shipping is done on-line through the post office website. It is not that difficult. I weigh all my packages, calculate the postage, print the label and give it to my mailman! It is called "Click and Ship" and the fee is put on my charge account, or use a debit card. The extra benefit of this is that it is CHEAPER to go online and weight & pay, as much as $.25 to $.45 a package. You can use this method for Priority Mail or Flat Rate. You cannot use this for regular first class mail or book rate.
I have been doing this for about 3 years and I love it. I have a large mailbox in front of my house on the road, and I just put the package in my mailbox and it is picked up.