I do have a problem with dictating how a charity spends their labor time,
Failing to inform yourself about how a charity operates and to oppose the sharing of that info is utter foolishness. To oppose identifying problems, exposure of them, and making improvements is essentially openly hostile to social advancement.
It’s sloppy and reckless to neglect to do some basic homework before selecting charities to support. If they are being needlessly wasteful, they are inefficient.
specifically in a way that would reduce their ability to provide services.
It’s the contrary. Sales facilitates provision of services. You’re advocating needless waste.
If OP offered their labor to handle this consistently for the charity, I missed that.
Probably what you missed was that a customer was in the shop with money and prepared to spend it, but couldn’t because the shop manager made a poor decision on what to accept from donors.