Answered by Mukesh
Whether several cards can be combined on one order is decided by the brand you are redeeming with, not by our store, since the redemption happens on the brand's own checkout. Some brands happily accept more than one code, while others allow just one per transaction.
If the brand does allow it, you usually enter each code in turn at the payment step, and the values add up against your total. Where only one code is permitted, you may need to spend the cards across separate purchases instead, which is a minor inconvenience rather than a real problem.
The terms shown with each card, and the brand's own help pages, are the places to confirm this before you plan a large purchase around it. Knowing the rule in advance saves you from standing at the checkout trying to make codes combine that were never meant to.