You have a few options:
Option 1) You can create a couple of line item 'coupons' with a negative value associated as non-inventoried and non-taxable. Example would be one for
-5.00 and one for -10.00. You could then print barcodes based on the ILCs you use for these items.
We actually do not coupon by line item because we use price changes with reason codes to track events at the HQ level across the board to properly allocate our ad fund based on trending.
The only issue you will have with just adding a line item is that it will not trigger the cashier for the coupon, the cashiers will just have to know to scan one of the codes when the transaction Sub Total (sales pre tax) hit a certain dollar amount.
Option 2) If this is something you will be working with frequently, you might think about looking into New West Techs BuyX/GetY program. This will auto trigger the discount with the Sub Total hits a certain amount.
Option 3) You can use the discount button and manually take off a certain $$ amount from the trx if Sub Total hits your guidelines. To pair off this idea, you could put your reason codes in place and back report against this - Looking at how many customers purchased with Code -5 and Code -10, etc. Plus with reason codes you can put a start and end date on when the code will be active and available.