Think about it. The offer was, "Give me the money, and I'll let her [your wife] go." These two guys had just been in mortal combat a few hours earlier, so they don't much like each other at this point. Even if Llewelyn cools down and accepts, how is delivery supposed to work? Anton will naturally suspect a booby trap, or an attack at a given location. The whole point is that both men have to have some awareness of a given time and place, which makes an attack highly probable.

The Big Lebowski suggests one possible answer: the aggressor commands the quarry to drive around randomly for a bit, then drop the package in a general area and keep moving. But even so, one doesn't rule out a booby trap.

The implication is also that Llewelyn is dead no matter what he does (whether by the other Mexicans or him), so why would Llewelyn even want to engage with a silly proposition. It was a bullshit offer with no reasonable way of being redeemed, even if Anton himself is too autistic to realize it.