I own and run a retail website and I ship almost everything by priority mail. What drives me super crazy is while I'm waiting to ship out my packages at the PO I watch people BUY their plain boxes/packaging from the PO and then ship their package first class. If you add in the cost of the packaging then it's almost the same price to ship priority, it gets there faster and you get the free tracking. I have seen them even buy the box and still ship it priority mail!
Priority packaging is free. Envelopes, boxes labels. It all comes with it free (as does tracking!) The flat rate options are the best if your products are heavy and will fit in the flat rate boxes. And it ships faster with priority mail (average is 2-4 days) than it does with the other options. I am NOT talking about express, which is over night shipping) that would cost double-quadruple if not more.
I watch the cashiers at the PO ringing up these peoples purchases and they don't say a thing about priority mail. It drives me crazy. I have tried mentioning it to other customers but they think priority mail is "express" and always say "oh I don't need it to get there faster" so I gave up. if someone asks I will point it out but if not then I just shake my head.
Here is my tip. NEVER completely box/tape up your package, until you get the rates from the PO. Sometimes it is cheaper to ship first class if it is going local and it is very light weight/small. Bring your shipping address and a roll of clear shipping tape with you to the PO. Stand online and get a price for both priority and first class or parcel. Then decide how you want to ship it. IF you already have the packaging then you can just bring the package and get the pricing. You can ship any package you already packed up priority mail without changing your packaging, but sometimes changing the shape of the box means a heavy package will fit in a flat rate priority box then you should unpack your package and stuff it in the flat rate box.
If I am not sure which works out better I go up to the counter with both a standard priority box and a flat rate priority box and then get prices on both and then stick my label on which ever is cheaper. I try to do as much prep as I can at home so I can get in and out quickly.