1. How are you receiving anything in return? Money that you could have spend on yourself (albeit with a bit more difficulty) and instead spent on צדקה, is your money given to צדקה. What's the difference if you give $10 to צדקה, or $10 amex credit to צדקה. Just like you can deduct a $10 cash donation from the מעשר money that you owe, so too, you can deduct a $10 amex credit from the מעשר money that you owe.
2. Even according to your presumption that you are getting something in return for Maaser:
If you have "The laws of Tzedakah and Maaser" (Artscroll), check pp 149 for a far more novel situation.
In case you don't have it, pertinent quote in footnote 31:
שו"ת אג"מ יו"ד ח"א סי' קמ"ג, דאפילו הנאה וריוח ממש הבאה לאדם ע"י גרם הצדקה שנתן שייך לו ולא לצדקה