It sounds like if you buy something that has S&S which normally offers 5% off, and use your Mastercard and enter code MSTRCRD1 you will get an extra 15% off for a total of 20% off.
I wonder if it will work together with the Amazon Mom that already gives me 20% off.
I tried testing it out but it's not working for me even on non Amazon Mom stuff like paper towels etc.
There was a problem
The promotional code you entered cannot be applied to your purchase.
EDIT: It looks like this deal may be over, and I believe it may only have offered 10% off which together with the usual 5% was 15% off.