So I ordered one book in a series through FIFO and, as I always so, browsed that member's shelves to see if they had other books I wanted.  Indeed, they had a number of books from the same series, so I PM'd her to ask if she would be willing to do a deal on the other books.  I never heard back from her, even though when I noticed that my original request was "accepted" I sent a follow up PM, saying I understood if the deal wasn't of interest but to please let me know one way or the other so I could order the other books through FIFO.  Now the book shows up as mailed - which is great, except when I go through FIFO for the other books I get the same person.  Now I feel badly ordering another book from this person since my original order has already been mailed and the sender can't save on postage.  Should I wait and let others order before me?

I think you should go ahead and order. As long as you said in the PM that you wanted to know because you would go ahead and spend the credits if a deal wasn't agreeable to her then it's her fault for not answering before mailing. Maybe she just didn't realize she could have saved postage. She'll still save some postage, though, since it sounds like your new order is for more than one again.

I wouldn't feel guilty.  If she wasn't smart enough to try and ship more books together that's not your problem. 

What Mary said.  I would just order.

Agree with Mary.  She may not check her PMs regularly, or she could just not be responding because she doesn't want to make any deals.  You don't have any control over where your FIFO orders go, so order what you want, when you want, and don't worry about it.

Thanks!