Understand this won't be super relevant to everyone but it was something I've been interested in and have a DP now that I got the Citi Premier.
Bought a $10 Starbuck gift card through the MPX app and it got 30 points on the Premier card. AmEx I believe still only gives non-bonus category earning for these purchases since they closed the 5x loophole on the Platinum card(used to earn 5x MRs for all purchases as it coded as United). I might try that one again once I hit the SUB spend for the Premier but hopefully this helps some people here now.
Edit: Should be noted that it was a DINING bonus and not an airfare bonus so depends on the store you're getting a gift card from.
That's interesting. I've been trying to figure out if I can earn anything extra on purchases made through MPX with my current cards but haven't found anything yet. Might be worth trying a little more. I'm just surprised they don't all code the same, regardless of what you purchase.