For WDW, pre-paying works. But it's less expensive at DLR, so don't prepay. Once you have the DLR photopass card ID number(s) in your account, you'll see the price for DLR, which for the past few months has been 59.95 plus shipping.
Definitely cheaper than last year. They started bringing it down around January, when it was about 79.95, and the 59.95 is the most recent, but like I said that's what it has been the last few months.
If you do go to WDW and want the link to prepay, just ask. It's not searchable at all on their site; you have to have the direct link to get to that page.
Here is the link, but like everyone is saying only use it for WDW as DLR is cheaper. I pre paid the 99, then when I got home found out about it being cheaper so got a refund on the 99 and paid the 79. And now it's 59? That's almost falling into the bargain category