There's no charge for them, and you don't need to show them any kind of 'proof' that you'll be using them for their intended purpose. They usually have them stocked in the lobby area, and you can take as many as you need.

They also have the cardboard flat-rate envelope. The post offices usually have on hand: Small, Medium, and Large flat-rate boxes. The small Flat-Rate box is usually too small for my purposes, but it may work for you. You can estimate the size you'll need and if your items will fit. If you check on the website, you'll see the measurements of the various boxes and envelopes. They're free, but I think you have to order them through and buy postage online. But I don't know if you can get them at the post office. There's also Regional Rate Boxes, which offer you a range of 'if it fits, it ships for one price' rates depending on how far it's traveling. The guy had seen the TV commercial, he knew what he wanted! Years ago, shortly after the Flat Rate Boxes were introduced, I overheard a conversation at the post office in which the clerk must have spent five minutes trying to get some guy to understand that a Flat Rate Box was going to cost him more than the normal rate structure.

If it's a light item that is not traveling far - within New York state, for instance - then USPS' normal weight-based rates are probably cheaper. USPS advertises the Flat Rate Boxes based on convenience - 'If it fits, it ships for one low price!' is the slogan - but this has gulled a lot of people into thinking that it's always a bargain. The one thing that I would emphasize is that it really depends on the weight and destination whether Flat Rate Boxes make sense for you. Others have already given you a good explanation of Priority Mail Flat Rate Boxes and how they work, but I just want to make sure I understand your situation: You live in Canada but you do your shipping by USPS out of the Niagara Falls post office? I know that many Canadians who live near the border do this because it's cheaper just want to make sure I understand.

Small flat rate box shipping cost