A cart is a wheeled vehicle. As such, the rules that apply to the drivers of cars and bicycles should also apply to shoppers. If you're going to text somebody a question about whether to get mild, medium or spicy salsa for the community theater after-party, pull your cart over to a spot where you won't be in the way and stop moving. The same rule applies when receiving texts — just pull over! Other shoppers have enough to worry about, what with dodging and weaving to avoid familiar faces, mopping up spilled wine, shepherding bestial toddlers and making sure the pile of shopping bags they brought doesn't fall out of the cart. The last thing they need are distracted shoppers weaving down the aisles with their eyes glued to their smartphones.
That said, judicious use of texting can save a relationship. If you're using a shopping list written by a partner and see something about potato chips, it's far better to determine precisely what flavor he wanted than risk the seething cauldron of disapproval that would greet you upon return had you made an executive decision and grabbed the ones that taste kind of like ketchup when what he really wanted was sour cream and onion.