https://www.law.cornell.edu/uscode/text/39/3001 (specifically, (d)) says that it's a scam:

> Matter otherwise legally acceptable in the mails which— is in the form of, and reasonably could be interpreted or construed as, a bill, invoice, or statement of account due; but constitutes, in fact, a solicitation for the order by the addressee of goods or services, or both; is nonmailable matter [...] unless such matter bears on its face, in conspicuous and legible type in contrast by typography, layout, or color with other printing on its face [...] the following notice: [...]

(emphasis added).

The law says that this wiggle room does not exist directly in the text. It's a scam, and clearly so.