INSTRUCTION

So often I go into a store, any store really, pick up what I came in for and head straight to the cashier.
"Will that be all for you today sir?" says my cashier.
Tell me, are these people trained to say this and for what reason might I ask?
"Will that be all for you today sir?"
"Well no actually", I reply, "I'm on my way to the supermarket to buy some fruit and then I'm probably going to run by the drug store to pick up some contact lens solution and while I'm there I might reach for one or two boxes of aspirin for this headache I feel coming on. I think once I have completed those errands that will most certainly 'be all for me today!'"