Fair enough - but if they're not being sold by weight, what are you actually 'checking'?
The weight of a pack/bag/box of whatevers, where the weight varies but the price does not.
If you're interest in merely in larger, or greater numbers of, items of fruit/veg, that's surely just a matter for your eyes, and your personal preferences, isn't it?
I'm not sure it's a strange personal preference to prefer to get 280g of tomatoes for £X than 250g. Nor am I sure that many people could tell the difference between the two weights by eye, or even by hefting.
I don't obsess over it - if time is tight I don't bother, and I don't lug entire cratefulls over to the scales, but if it's only a few steps from the shelf to the scales, why not pick up 2 or 3 packs of mushrooms and see which is best value?
I'm not sure what you mean by "presented with a choice of ...".
It's an equivalent example. If a bag of spuds is £1.30 and is nominally 1kg, but you have the reasonable option to pick out a bag which weighs 1.3kg, and you choose not to take it, then you are indeed choosing to pay more for your spuds than you have to.
You're being presented with a choice of bags of potatoes, all at the same price, and being provided with a handy way to easily find which bag (out of a few random samples) has the lowest (and therefore the highest) price per-unit-of-weight.
Were they sold by weight, and you were given a choice of price per-unit-of-weight you wouldn't tap the touch screen to pick the highest price, so why do nothing to avoid picking the lowest weight of a fixed price item?
I can believe what you say, but I don't think many of us would go to those lengths to check
The shop would not tolerate the converse. If there was a pack of beer on the shelf at £3 and you decided to just take it, and not pay them their £3, they would be, to say the least, aggrieved.
So why let them tell you that a bottle of wine is £9 but charge you £12 for it?