I added transfers and dining reservations using my computer post final payment and as luck would have it, the system crashed and wouldn't let me continue...come back later it said. When I came back on I found that the reservations processed but I was now no longer was paid in full. I immediately redid the payment and went back to paid status. So, I could see this happening inadvertently if you weren't paying enough attention which was part of the original issue...no documentation.
I should add that putting transfers in requires flight info etc so it's a lot more than just a single selection.When I re-entered the payment info there was no ability to edit any of the transfer info,so it was locked in.