Many carts today allow customers to filter products by size, color and other criteria.
However, where the shopping carts fail is that when on one of the products is selected that matched the defined filter criteria that the product options don't default to that criteria.
For instance, if filtering polo shirts by XL size and BLACK color and then click on a shirt showing in the filtered results, the default size setting should be XL and the default color shown should be BLACK.
Making the customer redundantly click on the size and color yet again is annoying, time consuming and a relatively simple patch to make to any cart. Probably wouldn't take more than a couple of hours to fix in most online stores.
If you guys implement this in your carts your customers will love you because it's a pet peeve that drives me and many others wild.
Don't think it's just the small vendors because many big box store websites have this flaw.