Yes you can get free Priority-Mail boxes at the post office. You can even bring them home vs packing it at the Post Office. The caveat here is that you MUST print a Priority-Mail label, so if this is something that can go First-Class, once you put it in a Priority-Mail box, you MUST pay for Priority-Mail.
You should get a shipping scale quick. It will pay for itself over and over if you start selling. Weigh things in the box, with the packing material, manifest/packing list, any notes/cares, stickers, and label. When in doubt to estimate, throw a few sheets of paper on top to account for the weight of a label and some paperwork.
There are four (4) types of Priority-Mail Postage, too:
Weighted. You simply round up to the nearest pound, so 1.1 lbs = 2 lbs
Flat Rate (Usually a very expensive way to ship unless it is heavy and/or going far.)
Region Rate (Usually a very easy and inexpensive way to ship especially if moderately heavy and/or not shipping far)
Cubic (Very much like Region Rate but pretty much your own box)
Save some packaging material from other websites particularly air pillows are very useful and light. Movement is what ruins most packages.
As others have stated, USPS will mail you supplies you can order online. Priority Mail boxes are free as are envelopes. You will want to order some plain Priority Mail boxes and some Region Rate boxes for sure. If it fits, it ships, so if a small box of some sort can go inside an envelope, that's fine provided it closes normally.
There are some seller secrets. For example, if it is going to Alaska or Hawaii it is ALWAYS zone 8, so more money. Flat rate is ALWAYS the same cost even if you mail it to your neighbor across the street who is really in Zone 1. Where flat-rate excels is that you can put up to 70 lbs in the box! That said it is usually expensive because the Post Office charges you for it to be somewhat heavy and go far. There is one small, two medium, and two large flat-rate boxes or 5 total Flat Rate box sizes.
A Region Rate box can be awesome. They come in A and B though there are two sizes for each. An A box can hold up to 15 lbs and a B box can hold up to 20 lbs. They more or less ship for close to about a 2 lb weight, so they are awesome and easy!
Lastly, there is Cubic, which is size instead of weight, but you are allowed up to 20 lbs. You multiply length*width* height in inches and divide it by 1728 (which is 1 cubic foot 12*12*12) in inches. If you get say 321 then 321/1728 = .1857 cubic Feet. It rounds UP to .2, so you would use the .2 cubic rate to mail this box.
You can use this for regular Priority Mail boxes, too, but they state on them their cubic size, and it is almost intentionally depressing. They will state numbers like .301 cubic feet, and you are irritated that it means that is a .4 cubic box where if it was .299 it would go .3 all day long and be the same size. It's almost intentional it seems.
