[Fred25]

And wonder if there would be any prints on these things. Guess the users would have worn gloves when they filled them, but probably not when they bought them.

Hmmmm...... the FBI probably photographed them with a camera before they were dusted for prints. If they contained prints, some of them might be the prints of the USPS worker who sold them to the customer.

[Cachelot]

If they contained prints, some of them might be the prints of the USPS worker who sold them to the customer.

That would tell where they were bought, at least - if they can match the prints. Time to print all USPS workers in some reasonable area?