I am working on an investigation of gun control and not police officer conduct, I would advise the officer that taking of gratuities is not allowed and I won't be a part. I would also note it in my file as part of the days activities reporting it to my chief upon return
to the department. I would not participate or enter the store in question.
I am concerned the officer just probably wants to save a buck, but this could lead to more advanced forms of bribery. Being that we a looking into a gun advocacy group I would concentrate on that issue, and do a complete and in depth job concentrating on the assignment. I would bring it up to my superiors upon returning to my department and let the department take what ever action they feel appropriate.