Recently I noticed that a couple of Airline Shopping Portals are now featuring Marriott as an option to earn extra miles for Marriott stays. This includes both American Airlines & United Airlines.
The amount of miles you can earn per dollar occasionally change. I've seen offers of 2 miles per Dollar and 1 Miles per Dollar at different times. The offer appears to be valid only for Marriott Brands (excluding SPG).
Here are the two links:
American Airlines: https://www.aadvantageeshopping.com/me____.htm?keywords=Marriott&gmid=2421
It's always nice when one can Double Dip!!