Yes - you have to post the book to your shelf though (I am pretty sure there isn't a way around that, I could be wrong).  Once the book is on your shelf, go to your bookshelf and click on the book. From there, you'll see a "send to a friend" link on the right hand side and you can type in his/her address.

The only problem you might run into (probably not with this book but maybe with others) is if the book is on someone's wishlist...if that happens, your book is automatically offered to the first person on the wishlist and you wouldn't be able to send it to your friend.  If that is an issue, I'd suggest buying postage online through Paypal, if you have a Paypal account. 

Another workaround for using PBS to mail out WL'd books to a friend.

First, I check the ISBN of the book I want to send out to see if it is WL'd.  If it is, you cannot post it to your bookshelf to use the Send to a Friend option, because it will immediately enter the FIFO system and be tied up being offered to the first person in the WL line.

So, instead, find a book with many copies posted in the system. (Community, Top 50s, Posted tab)  Find a book with hundreds of copies in the system to use as a "dummy" book.  Post the ISBN for this book to your bookshelf, and then use the Send to a Friend option.  If you use the Printable Postage option, as I do, make certain to change the weight of the book to match the weight of the book you are actually sending, and not use the weight of the "dummy" book.

The only drawback to this method is that you don't have the actual book name in your records,  so if you've used this option more than one time, it is difficult to remember which actual book each "dummy" stands for, but the transaction record does record where you sent it and to whom, so you can track your transactions that way.

Edited to note, I also use a personal tag with actual book notations to remind me which book(s) the dummy book actually is replacing.

Another PBS member is helping me to do that and this is what she wrote:

No you don't post it first. I just tried it and here's what you do. You would go into my wishlist and scroll down to (name of  book). Then you would click on "Post This Book" next to the correct title. It will come back with a screen that looks like you are posting it into the system but there will be a note over the book title that reads 'You are posting this book to this member: (name  of member)