It seems to me that M-W is here conflating and confusing two separate instances, and the 'especially' implicitly acknowledges that they realise this.

'3 items or less' is descriptively OK, because that's what so many people, from supermarket signwriters to paying punters, actually say.

But it remains prescriptively wrong, because in supermarket terms, you can't have a unit smaller than an item*. <3 items = 2, 1 or 0 items.

But that is not the case with 'dollars' or 'miles'. $19.99 is less than $20 because you have removed one cent, not one dollar.

'Fewer than 3 miles' would (properly) refer to miles only as discrete integers, meaning either exactly 2 miles or 1 or 0, and would be incorrect if you meant a range which included distances measured also in subunits. Will no one think of the chains and furlongs??!! The yards are hard enough to take care of themselves, of course.

*I mean, you can, if you split a six-pack that was only meant to be sold whole on special, but then you're breaking the rules of Walmart, not grammar.