Of course they loose some money on the gift cards, but I highly doubt it's anywhere close to 18% of the value for most retailers. I was not able to find a public source on any numbers here, but I asked a friend working at a large 8-letter games store and he mentioned figures in the range of 5%.
Furthermore I am not speaking of the recently launched game store, but the UE Marketplace that sells assets targeted at developers.
As mentioned in a reply to a sibling of yours, I am not aware that Steam or other large game distribution platforms are competing in this market.
Hence I am not sure why their rates are relevant here.
This might be a regional thing, but I have never seen iTunes gift cards discounted where I'm from (Germany). What I've seen is that if you buy a gift card at face value, they give you an extra gift card for 10-15% of the value. And I've not once seen such a promotion for any non-iTunes gift card.
This approach leads me to assume that that either happens in co-operation with Apple, who are eating the extra cost for the special promotion, or the stores are paying out of their marketing budget for a loss leader.
I don't have any experience with the gift card markets in any other country, so things might just be different here.
Either way this is all hypotheticals since I still can't find any good hard numbers.
EDIT: Of course you also need to take into account that not all sales in the stores are paid for though gift cards.
