This just in...(well its news to me and haven't seen anyone here talking about it)
Apparently its been confirmed that 20% off codes that started with YJ stack with 20% off codes that start with YT.
That means you can use 30% from amazon mom, a YJ 20% coupon, a YT 20% coupon and a $10 off coupon for a total of 70% + $10 off