We did this for our trip last August. You place your order online at the discounted price, and they send you a confirmation e-mail with the code to "purchase" the actual CD when you return home. Note: don't delete the email and don't lose the code.
You pay upfront when you place the order, then you have 30 days (I think it was) from when you return (actually, it seems it was thirty days from the first time I entered the code from the photopass card the photographer gave us). Somebody mentioned that it is good to wait in case you decide not to do it after you get there, but that isn't an issue because satisfaction is guaranteed, so if you choose not to use the service, and don't use your code, you can still get your money refunded.
When you finish adding borders, etc. to your pictures and you are ready to order the final CD, you simply put the code in when you go to checkout and it zeroes your balance since you already paid up front.
You can actually do it the day before you go, or even while you are there. I think the two things that will determine it for you is that you need to do it before you ever get home and enter a code from a photopass card, because then you didn't "preorder" and also you may not want to wait too long after you get home to do anything because they don't keep the pictures available forever. I don't know how long they are available for you to view if you don't do it fairly soon after you return home.
HELP!
). Make sure you do ALL of your edits, etc FIRST. When you are sure you are done, click on the "products" tab at the top of the page, select the photocd. It will tell you how many images are ready for your CD, click "add to cart". Then go to your cart and enter the code you got in that famous DO NOT DELETE email. Viola! All you have to pay for is shipping. I chose standard and it was $7. 
