In powerstore you can browse and pick items for coupons to apply to.
If you are doing this manually in eCart and don't have a database driven coupon system like what powerstore uses, then you would probably have to do it manually.
You could add a column to your shopping cart for "coupon_applicable" and set it to "1" for items that are included in the coupon and "0" for items that aren't. Then you can create a calculation for:
discountedTotal = [Quantity] * [Total] * [coupon_applicable]
Then base your coupon on 0.2 times the total of that calculated discountedTotal column.
There are really a million ways you could do it depending on the complexity of what you are doing. Do you have multiple coupons like this each that apply to different groups of items? How often do the coupons change? At some point couponing can be very complex, but it all depends on the specific details of what you need.
