There is a field to type in any amount you want up to $500. Since you're getting 10% off and you want to use as much of the $100 giftcard dotcom GC, I was buying $109 DGCs which results in a price of $98.10, plus $1.99 shipping = $100.09, meaning I use the entire $100 giftcard and then have to pay an additional $0.09. But since I have to do 3x transactions, I guess I started to set off fraud alerts for making 2x$270 transactions followed by 3x $0.09 transactions. It might be better to use PayPal and use your bank account for the $0.09 transactions.
Because of the issues I was having, for the last 2x, I just purchased $108 and left $0.81 on the last 2 $100 gift cards.