The MPX app from United is convenient way to rack up bonus United miles for purchases you’re making regardless. You’ll net an extra 1-5 miles per dollar (sometimes more) by paying with a gift card bought via the app instead of paying direct.
The question often arises whether the category bonus from the credit card will carry over on an MPX purchase.
I conducted a bunch of experiments to learn more, and here are the results.
Chase
Update: Seems Chase is now coding MPX as generic purchase, and it stopped earning any category bonuses.
- CSP Starbucks  – earned 2x as restaurant
- INK, Office Max – earned 5x as office
- INK, Staples – earned 5x as office (There are conflicting reports about this on Reddit here and here; my recent experience is that it worked; I’d guess that it used to not work and now does work.)
- Uber – works for 3x on CSR and triggers travel credit
- Freedom, Kohl’s – did not work for 5x as Department Stores (despite the fact that Kohl’s is specifically listed as an eligible merchant with Chase)
- Freedom, Sears – worked for 5x as Department stores (though using a portal to buy the gift card might be a better idea)
- Hyatt, Papa John’s – earned 2x (heard from  a friend)
- CSR, Red Robin – earned 3x as restaurant (heard from a friend)
- CSR, Domino’s – earned 3x as restaurant (heard from a friend)
- Freedom, Panera – earns 5x for rotating restaurant category
Citi
- Prestige $10 Starbucks gift card – earned double points for dining
- Dividend, Kohl’s for 5% in the department store category – this won’t earn 5% (from Citi rep)
- Dividend, Sears for 5% in the department store category – this will earn 5% (from Citi rep)
- AT&T, Staples – earned 3x
Amex
Amex lags a month with issuing rewards, so I won’t know for another few weeks whether these worked (I don’t think they will). I’ll update when the results come in.
- Platinum personal, Staples – pending (waiting to see if it gets 5x for Airfare)
- Business Gold, Staples – pending (waiting see if it gets 2x for Airfare)
- SimplyCash, Staples – a friend confirms this did not earn 5%
Discover
- Discover, Kohl’s – didn’t work.
- Discover, Amazon – didn’t work
- Discover, Sears – worked
Barclay
- Sallie Mae, Amazon – does not earn 5% (pic)
Final Thoughts
As a rule of thumb, it appears that MPX accurately captures the category coding, but won’t get the store coding. So generally, you’ll get the bonus category for dining or department stores and similar, but you won’t get the coding of a specific store (e.g. Discover or Sallie Mae for Amazon). There are exceptions, such as Kohl’s, which is usually considered a department store, yet MPX doesn’t consider it as such.
It’s impressive that MPX gets the category coding correct, something we don’t typically see. I’ll update the Amex and Discover experiments when the results come in, and please add your own data points in the comments!
