Less than/fewer than/under

Okay, I'm struggling with something that seems really simple.

Although every effort is made to deliver orders by the requested date, we cannot guarantee delivery in less than 2 business days

I'm the one who rejoiced when the supermarket changed its signs to read "12 items or fewer," so it's not that I don't know the difference between fewer and less.

But I'm considering "2 business days" as a block, not as two countable days (since it could be four days if there is a weekend in the middle) and think that "less than" works. I think that "under" is too informal. I think that "in fewer than" just sound odd in this case.

Am I losing my marbles? (Please, American experts - this is a local thing that doesn't need to work world-wide.)

Thanks guys!

Amy, thanks!

This is just what I was looking for to show (myself) that I wasn't totally misguided: Less than can be used before a plural noun that denotes a measure of time, amount, or distance.

Okay, I'm happy about my sentence now.

(What I really wanted to write was "Look, bucko, if your act wasn't together enough so that you only ordered them the day before you had to actually have them in hand, then I may not be able to help you!" But somehow, that didn't seem the right way to phrase it. Nonetheless, it was cathartic just writing it here.)