If you are close to the location where the USPS hands off to the Military system or ship's homeport, go Parcel Post. You could check to get an idea of how long by adding a "Delivery Confirmation". It will show when it hit the handoff point or homeport. That will give you information for future decisions.
If you're mailing from a Main Post Office in your city, go Parcel Post, especially if it's a small to medium sized box.
If you're sending a package on a regular schedule, send all Parcel as it's only the first one that seems to talk longer.
The money you save can buy more stuff to put inside the box.
"Delivery Confirmation" will not track mail once it's in the military system. But you can find out when it hit the military system. From that point on, there's not difference how mail is prioitized for delivery with regard to Priority vs Parcel Post.
That's good advice about shipping Parcel Post. Freepers should try to be aware of things which will save them money on shipping.
Here's what my experience is when I ship something APO. Suppose there is a need and Patriciaruth sends out a request to the Freepers for pillows. I will leave my home and find a store with pillows. Then I will select a box for shipping. Most times I guess wrong and get a box that it is too small, requiring me to get another box. It is not a problem if I get one that's too large, because I will find other things to fill it with. After finding the box, I have to wrap it , and then, I go to the post office, fill out a customs form, and stand in line.
The rate for shipping is pretty steep because the package has to travel across country.
Suppose though, that I shop online and I am lucky enough to find a company which will ship the pillows for me, and it happens to already be in or near New York. Ah Ha! Now, I don't have to leave my home, buy the pillow, find a box, wrap it, go to the post office, fill out a customs form, and stand in line. Additionally, I have just saved time, gas, and mileage and two to three thousand miles of shipping cost because the company is much closer to if not in New York!
Conversely, suppose Patriciaruth needs to ship something FPO, and she is fortunate enough to find a company online which has what she wants, and the company is located in or near San Francisco. She gets the same result. She doesn't have to leave her home, find the product, find a box, wrap the box, go the post office, fill out a customs form, and stand in line. Additionally, she saves time, gas and mileage and two to three thousand miles of shipping cost!
My initial motivation for posting this was to minimize some aggravation for Freepers who need to ship something APO/FPO. It dawned on me after I posted that it can also save a LOT of money if the company is located near the place of shipping.
And, back to your suggestion of saving money, this could save Freepers a LOT of money, and that is more money then that can be spent on the troops! :)