Fuzzy logic dude. I take it finance wasn't your major in college. Most earnest people will purchase a single garment they think is the right size based upon sizing charts that are provided by most sellers as things like jerseys and bib run to a particular sizing convention. Also reviews of products help solve this for prospective buyers. The probability therefore of a savy buyer returning an item maybe 1 in 7 to say 1 in 10. In other words they will get it right most of the time. I buy everything on line.
Your premise is flawed. Returning items isn't a certainty only when an error is made by the buyer for choosing the wrong size. We have all returned things.

I also know people who go to home depot and buy a hedge trimmer, use it on their house and return it. I put you in the same category. If you don't see this...even worse...lol.