I tip just about anyone for anything, I can't help myself and my daughters say I'm mad but I like to think I'll get good service next time. I always give the Asda man £2, it's not a lot and it pleases us both.
With Asda, if you don't like the substitutions you can say no thanks (but if they give you something bigger or better you only pay the original price).
The last time I had a delivery I wasn't very well. The driver could see this so he brought all my groceries into the kitchen and unloaded all the freezer stuff for me.