Call and ask the are a couple of factors:
Adding the DP is a change, so the change fee could be applied, Disney may or may not do this. There are reports that they've gotten tougher with this, but I'd love to hear your experience.
Secondly, you usually have up to 72 hrs prior to check in to add the DP so in theory you could wait until close to your trip (I'd do it no less than 5 days just to be safe) and even if its on the spot payment you have time to arrange that.