I've also worked food service, and yes, I usually tip 20%, but not for counter sales, and definitely not at service stations. Asking me for a tip at a non-service (I.e. other than restaurants, or hotels), I take as a demand that I change to another business, and I usually don't come back.
My only beef with self checkouts are places that have cut to measure items, or heavy items that don't fit on the auto weigh sale counter. The whole sale grinds to a halt until an associate comes over to sort it out.
I tried preordering online and then picking up at a few places, but I find it hit or miss. Once, my order got split up into three batches spread over seven weeks, resulting in my having to go three separate times (an extra hour and an half each time). Another time, I ordered a dozen T-posts as part of an order, only to have the associate try to tell me that the price covered twelve bundles of ten. Twenty minute delay while the associate took the whole order back to check with a manager, before coming back with a dozen T-posts. What happened to the snazzy radios that they use? The last time I paid $0.60 for a 7' T-post was never. You have to laugh sometimes.
All the best,
Peter
