These stickers are applied by the PO and it wouldn't be unusual for a small franchise one to 'try' it on to see if they can get away with it .

My local PO does this which usually occurs when a AP staff member stands in for the owner ( who is often absent) 

That aside , 70c standard mail should never contain anything else but written material . As a buyer I'd be pretty annoyed to receive items like this as they are easily caught into the sorting machines .

Charge the appropriate rate ($1.40 for you) plus a padded envelope and you won't have any issues .

The amount of times I've had beads/findings sent to me in an letter envelope and 70c stamp and it's been damaged/contents lost etc is beyond a joke.

Sellers know better , anyone sending mail like that are asking for trouble just to save yourself a few cents . And if your return adress was on there you would get the bill , so if you don't put your return address on there IMO your doing it on purpose.

Just my opinion