The all natural way of dealing with ich is hyposalinity. However, I don't see how it would be practical for a LFS do hypo on their fish, just because of the acclimation process.
If you do use copper, then use Cupramine, maintaining copper levels at .4ppt using a Seachem copper test kit to ensure that that level is maintained. Cupramine is a relatively safe copper, less harsh, and effective. Maintain very good water circulation and good water quality.
If you follow this process, at least you will actually eliminate the ich problem [if you keep your new fish for at least a few wks]. In the long run, I would think that customers will appreciate getting ich-free fish from your store, and reward you by patronizing your establishment.
Good luck!