We'd been in sort of a limbo, cart-wise. The kid used to stay in his baby bucket, which would be snapped out of the carseat base and clicked onto the metal seat-back of the grocery cart (trust me, it works great) to be wheeled around Target or the grocery. But then he outgrew the bucket (actually the weight of him + bucket just got to be too much for my wrists) and we switched it for the Britax giganto carseat which does not snap out because it weighs four million pounds and is the size of a Volkswagen. So then he had to be carried, put in the Ergo, or strollered -- none of which work terribly well for Target and the grocery, especially when the adult is flying solo.
But then he started sitting up, and we thought, "Woo-hoo! He can sit up in the grocery cart like a little kid!" Except he hated it and made his feelings known in no uncertain terms (the terms in question being sustained screaming). Until today, when suddenly the cart was A-OK. He sat up, looked around, chewed on the cart a little (T-minus a cold's incubation period ... ), and was perfectly content to be wheeled around in state as befits an adorable despot.
Which just goes to show: never stop at once. The first time for everything almost always goes badly. Hell, the second and third time probably goes badly. But at some point, without warning, the kid will change his mind, and suddenly the avocado/grocery cart/cloth blocks will be a big hit.