The other day I noticed a handmade sign at my local Post Office. Someone had posted “PLEASE PAY FOR THESE SUPPLIES AT THE WINDOW COUNTER. THANK YOU!!” on a display of boxes and mailing labels a few steps from where customers wait in line. The entire sign is scribbled with yellow highlighter. That kind of thing usually indicates a problem.
The USPS provides flat-rate mailing boxes free of charge in an adjacent display, and I suspect people were innocently walking off with the more specialized shipping supplies without realizing it was a problem; I can easily see someone using a label off the rack to address their parcel. Providing some resources for free seems a little arbitrary, but the main problem with the retail display is that it doesn’t show a price for any of the items.
It should be a pretty obvious pattern, but showing the price is a good way to let people know they should pay.