I've just ordered a book for a friend, which she had requested.   The member I ordered it from has other books on his/her bookshelf that I think my friend might want, but I don't want to order them unless I'm sure.   I can't get in touch with my friend just now, so my question is: How long is the window of opportunity for ordering other books from this member?

I think you have until the member prints the label so there is no way to know for sure. 

Why don't you Pm the person and let them know you are waiting to hear from your friend  about possibly ordering another book or two and ask that they don't print the label and package it up just yet .

What Kendra said 

I would send a PM like Kendra said. I would then wait to print the wrapper if someone sent me a PM. Otherwise I try and print the wrapper asap because I have this weird irrational fear that they'll cancel the order, and I won't get to send the book (and get a credit).

Thanks, all.  Friend just wrote that she doesn't want any other books on the member's list, so panic averted. 

It's really nice of you to help your friend and I realize you said in another thread that for whatever reason she can't mail books herself.

It does seem to me that it would be easier for you if she just opened her own account and did all her ordering etc herself.  She isn't required to send books out - she can buy credits. Or, if you are close enough to her to deliver the books she is ordering through you (I don't recall if you said) you could just pick up any books she wants to swap and mail them when you mail your own.

You don't need to explain why this would or wouldn't work - it's just an observation.