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If the book is WL-you can post a "dummy" book to your shelf and then Mail It To A Friend. I used to use a book from heavily posted author I don't read.
If the book isn't WL-then I just post it to my shelf and immediately Mail It To A Friend. That removes it from your shelf and then you can print the label/postage when you want.
Then you just have to mark it received when they get the book. It'll appear in your Books Sent and Books Enroute tab. Probably because they figure the other person is not a PBSer and can't mark it mailed.
I used to do this all the time with game books when I didn't have time to go to teh post office.