Ok. I tried tonight.
Here's the catch.
You can have 20% and 15% on the same transaction but not the same item. BUT you can NOT stack the $xx of $xx coupon to the same transaction.
So the best bet will be 2 legos and use 20% and 15% in 1 transaction.
I can use the $15 off $100 on Dimensions pack, but no other coupons.
and yes, I buy 12 3-CPO and get 1 FREE.
I tried use the $15 off $100 even after my discounted lego (1 with 20% and 1 with 15%) are over $100, the system will throw you an error saying no enough amount.
I tired used the $5 off $25 even after my discounted lego (1 with 20% and 1 with 15%) are over $25, the system flag the coupon as OFFLINE, I have no idea what that means.
In Oct, we can have $5 of $25, 20% and 20% on the same transaction, but in Nov, that's the case anymore.
It still can be done, but need manager override, so I tell them remove the $xx off $xx coupon, and everything works.
Hope it helps.