I asked Apple Support about it the moment I read about this offer in the Apple Store app and they assured me this would work... for those of you who already have some Apple gift cards saved from birthdays or holidays or whatever, you are allowed to use the gift cards you already have as the payment method and still receive the promo gift card in return as long as you don’t add any other gift cards to your cart after adding the promo gift card.
if it is something on the cheaper end of the scale you’re interested in buying, I personally recommend you stock up on $100 Apple eGift Cards from Target online as they are doing a similar promotion except they give $20 Target gift cards as part of their promo.
I’m gonna get the iPhone SE 128GB variant for $449 and just under $500 after tax so I plan on stocking up with 5 $100 Apple eGift Cards for a total of $500 from target online to receive 5 $20 target cards totaling $100 value in Target gift cards. Then I will use the $500 in Apple gift cards as my payment method in the Apple Store app and still receive $50 promo gift card in return...
This method will probably not work all the way with higher value items like the MacBook Pro or iMac or iPad Pro since Apple limits using up to 8 gift cards to pay