If I do this for you, you need to pay me the cash for the cost of the groceries you want in advance.
I can give you the change and receipt.
You probably need to come pick them up at the house, unless your location is super convenient for me and already on my way to somewhere I need to go.
(I am putting this category as Help At Home > Miscellaneous, because I'm not giving transportation to someone who needs to run an errand like shopping. I am physically doing the shopping. Therefore, Transportation for Errands including shopping didn't seem accurate as a Timebankers' Talent category.)