This is journalism at its finest: savethestudent.org/student-deals/food-and-drink/free-10-costa-gift-card-with-5-mastercard-purchases-via-samsung-pay.html Copying my OP and changing a couple of words (know this because the information about eligible banks and the how to redeem title are in the same place). Would be nice if they bothered crediting me!
From the OP: MasterCard's from MBNA, John Lewis, M&S Bank, HSBC, and Santander are eligible. Cheapest item meaning by you could buy 5 x 5p bags with Samsung Pay with those MasterCard's and get the £10 Costa e-gift card
Never bothered with Samsung Pay iv always used Android now Google
Quite disappointed to see very few banks actually participating in the Samsung pay app