Yes, you're 100% right. USPS regulations state that first-class “letters” may contain only paper and a fastener (ie: staple, paperclip). No coins, seeds, buttons, pills, or whatever you might send.
If a “letter” contains any object other than paper and fastener, the "letter" is now a “parcel” and must be shipped by a parcel rate (including the new “Ground Advantage” rate). This starts at about US$5 depending the weight, thickness, girth, etc. The counter clerks have a cardboard sheet with slots that they pass the envelope thru to determine its girth and flexibility etc and they're supposed to bend the envelopes looking for objects to determines the envelope's class…
But it gets better! I once sent a “parcel” that was TOO LIGHT to qualify as a parcel but contained an object (a coin) so it couldn't go as a letter either. The clerk rejected my letter / parcel altogether!! So I put 2 stamps on the envelope and dropped it in a mailbox. The letter and coin were safely delivered a few days later.
The regulations are not new but have not been enforced till the last few years. So in addition to the possibility of theft when mailing coins in a first-class letter (it's not secure), you now have to worry about your letter being returned - if you're lucky enough to ever see it again. Whew!
Moral of the story: I'll send one or two low-value coins by 1st class mail and cross my fingers, but anything worth more than a few bucks goes by Ground Advantage or Priority Mail, both of which include insurance and tracking info. But the USPS and EU are really hurting our hobby.
PS: back in the 1800s book publishers forced the USPS to create Media Mail with discount shipping rates. If only we could band together and force the USPS to adopt a poor collector hobby rate??