Update: November 2, 2017
I guess 3.5 pts / $ at Amazon.com was too good to be true. Last I checked, Amazon is no longer listed as a retailer.
Edited November 1, 2017 to correct actual earning rates in the conclusion.
Once you sign in to your Marriott account, the MORE portal shows how many points / $ you can earn. This value appears to take into account all bonuses you are eligible for. The only retailer that stood out to me was Amazon.com. Being a Platinum member and having the Marriott Rewards Premier credit card provides a 50% and 25% bonus respectively. I think it’s better to use a different shopping portal for the other retailers, but this is the only shopping portal where I’ve seen a reasonable value for Amazon purchases.
To take advantage of this, you need to install a browser extension or install the MORE app.
You start you checkout process like normal, but when you get to the Checkout page, you need to click "Pay now" to select Marriott points before clicking "Place your order".
I finally downloaded the app (which is not available natively for an iPad). Here I was able to complete my transaction with minimal issues.
For the $56.98 charged, I received 114 base points (2x pts / $), 57 Platinum points (50% bonus = 1x pts / $), and 29 credit card points (25% bonus = .5 points / $) in addition to the 5% back from using my Amazon Store Card. This results in earning 3.5 pts / $. I figure points are work approximately $0.01 each which results in 8.5% back at Amazon.
I figure it’s worth the effort to add items to my shopping cart until I am ready to make the actual purchase through the MORE app.