Line item discounts allow for you to run deals in a more effective manner and not discount the entire cart (aka - make more freaking money!)
How to do a line item discount.
Add your item to the cart (duh)
Tap the item in the cart to see the Line Item Modal (bad name)
Here you can apply an existing coupon to just one line item
You can also:
Adjust quantity with arrows (much better than the old way).
Remove all quantities of the single product from the cart.
