https://bestbuy.cashstar.com/gift-card/buy/Maximum of 3 $150 e-gift card purchases during promotion time period per customer and savings codes will be sent via e-mail in $15 increments.
So i assume you can get the $30 back with Amex offer to make it officially a $60 off $300.
EDIT: its thru cashstar, so not sure if itll work