Only one discount can be applied to an order at a time.
BUY 2 GET 1 FREE is automatically applied if you qualify. You can not apply a discount code generated from your Comic Fragments if a discount is already applied. If you attempt to do this, you may get an error message and the Comic Fragments will not return to your total. However, the code generated from the Comic Fragments can still be applied to another order. You just need to ensure that other discounts are not applied already. You can find the code you generated by clicking on the red circle with the white star and opening your rewards.
Gift cards do not count as discounts and can be applied to an order with another discount.