Geez, if all those fish have ich comming in, all the time, maybe you should look at switching dealers.
Ich treatment takes time. There are only 2 viable treatments, copper and hyposalinity. You have to treat the fish with one of these to be effective. Both treatments are going to take about 3 wks.
I suppose if your fish tank filters, water, etc., don't mix with your inverts, LR, etc., you could run copper in your fish tanks all the time.
I would encourage you to promote QT setups to your customers. Even if you don't sell the QT yourself to the customer, such as in the case with someone who uses a rubbermaid to setup a QT, I think you will get the thumbs up in the long run because the end result of the QT will be healthy fish, thus happier customers.
You could put out a 1 page flier, for instance, that explains the benefits of QT and the options for setting them up.