USPS has 3 types of First Class mail: letters, flats, and packages. The criteria for each category are very specific; each piece falls into just one category, they do not overlap. The criteria include weight, dimensions (including thickness), flexibility, etc.
If your envelope falls within the size and weight limits for a letter, you can mail it very cheaply. A letter can't be more than 11-1/2 inches long, or more than 6-1/8 inches high, or more than 1/4-inch thick.
If your envelope is too thick to mail as a letter, you can mail it as a flat if it's not over 3/4 inch thick. Flats must be flexible and literally flat (variation in thickness no more than 1/4 inch). Postage depends on weight; for 2 ounces it would be $1.20.
Any piece that is too big or otherwise doesn't fit into one of the first 2 categories has to be mailed as a package. USPS provides tracking for packages but not for letters or flats, so some sellers deliberately make their packages over 3/4 inch thick so they can get tracking, which protects them against fake "Item not received" claims. Some categories are more vulnerable to this. I suspect that masks is not a high-scam category. For any package weight up to 4 ounces, the online postage cost would be $2.74 - $3.18 depending on distance.
