Yes priority mail is expensive and if your lucky will only get your item there 1 day faster. I ship all my bubble mailers first class in the united states and Air Post if shipping outside the USA.
Shipping first class in the US is between $1.11 and $1.78 And Canada Is a Flat $1.78.
I have never heard of Free shipping supplies for First class.
When Me and my wife sell clothes we sometimes. ship using the Flat Rate Ebay Priority mail boxes. But a note: the flat fee will in many cases be more to ship with the flat rate boxes then using your own.
Since aside from my ebay and salehoo ventures, I also work with a resturant 15-20 hours a week I aquire free shipping boxes all the time through the resturant. You can pretty much aquire free boxes from anywhere.
If you have a Kroger Supermarket , Or some other 24 hour store you can usually walk in while they are stocking the shelves and they are more then happy to let you take all the boxes you want. Keep this method in mind so you can have that perfect size box for your items.