On their update FAQ:
5. I have completed my order, but I forgot to add some items
Please, write to us at email@example.com.
We will cancel your order and add amount of coupons that you had spent on this order, so that you can make order once again.
Please note, that it may take a day or two - but we will get to your email for sure.
That's the general 'help' email for pledge manager issues.