I did not "insist" on small amounts, I pointed out that if you based your search on large payments, you'd miss the small purchases.
What I am telling you is it doesn't matter. Big or small, when the government sees every transaction they will be able to untangle your operation.
You would indeed need to perform UA's on all the customers of dealer to catch all of his customers, and I'd point out that there are drugs which do not show up in UAs, or which are processed through the body and out again long before any such test can be performed.
By far the most commonly used dangerous illegal drug is marijuana, it is also the easiest to test for and the
most profitable to criminal cartels. Drugs such as cocaine and heroin and meth are addictive enough that the customers tend to be daily users and will only be evading the test if you warn them of it ahead of time (which you aren't doing), and even then they might not be able to manage.
And yes, selling drugs out of a drive-through window is probably a bad idea. But that doesn't mean that trusted customers to a bodega-type store can't be sold the tea they keep behind the counter.
Again, you are not the first drug abuser who has tried to figure this kind of thing out. Code words, trusted people, whatever. It's not sustainable, you get busted.
Oops, you messed up just one time and gave someone the wrong bag. Busted.
http://articles.cnn.com/2002-10-10/justice/ctv.scm_1_bank-robber-marijuana-hotel-pool?_s=PM:LAWA fast-food restaurant employee was arrested September 21 after a customer drove up to a Kentucky Fried Chicken drive-in window and received the wrong side order with his chicken dinner: two bags of marijuana.
Doing a UA on the entire customer base of such a store would:
If there is any significant non-drug using customer base, the owner is a weapons grade idiot for not just catering to them instead of risking his business to sell illegal drugs when all a bust takes is one simple mistake, one careless employee, one person randomly getting pulled over when a cop saw them leave your store and getting caught with the drugs, and a billion other things that can go wrong even before you bring to the table the fact that all your transactions are being watched.
Prohibition doesn't work
It works when done right, even in countries without Regulated Bitcoin Economies. Look at Singapore where drug use is very low and harsh laws are enforced.
http://www.guardian.co.uk/commentisfree/2010/jun/05/singapore-policy-drugs-bay