The Goq-
A quick check confirms that you’re correct – as a first party collector, the “Fair Debt Collection Practices Act” doesn’t apply to the store owner.
Thank you for the info.
The site at which I confirmed “The Goq”s info further states “First-party collectors are regulated by your individual state law which, obviously, varies“.
(Since this case involves heimishe people, it would obviously be best settled in a halachic manner by the customer and store owner.)