I just want to throw in some support for this idea. It sounds like a superfluous modification, but there are a lot of people who balk at creating a membership if you ask them to do it in advance, but if you ask with a simple checkbox once they've almost completed their order, they would say "why not? It's easy and they've already got my info..." Assign them a random password through email and immediately invite them to change it by giving them a reset link.
Making the membership decision at the end turns membership into an impulse buy instead of an upfront decision. It's just a much more attractive flow.