This seems like an easy way to earn Membership Rewards points for minimal effort and this promo has a rather extended deadline. (flyingfromthefront.com)
If you prefer, you can simply earn Membership Rewards points for those purchases and use them on travel, which is usually more valuable. (rewardexpert.com)
I've never done a paid Marriott stay so I'm not very knowledgeable about this, but I do know that you earn Marriott Rewards points on your paid stays. (plane2port.com)