QuickBooks Point of Sale discount code

Some of my items use different units of measure. I'm trying to input a coupon that would give $15 off a customer's entire purchase (which is reimbursable to me later). The reason I'm using a coupon rather than a discount is so I can track the coupon for an entire sale rather than item by item. You cannot enter a reason for discount when discounting the whole sale. For example:

If someone purchases one item for $16.95, inputting a discount for $15 on that item with the reason "Promo" will allow me to track that sale for reimbursement.

However, if someone buys 3 items @ $6.95 each, I will have to discount each item with the reason "Promo" until the $15 is depleted:
Item #1 - Discount $6.95
Item #2 - Discount $6.95
Item #3 - Discount $1.10
Total of $15 discounted (customer's remaining balance would be $5.85)

If I use a coupon (with barcode or the like) it will take $15 off the entire order once everything has been added to the sale. I can also run a report to see the total for reimbursement.

The problem I'm running into is when I add all items in inventory to the coupon, only the base unit of measure will show. There is a note to use the "Unit of Measure" column to change this, but that column does not show when selecting items or editing the list. When I try to use the coupon code on a unit of measure that is not the base, I get a popup saying the item is not eligible for the coupon.

The only workaround I've found is to input a $15 discount with the reason "Promo" on any one item. This does discount the entire sale $15, but I'm afraid what it will do to my cost of sale with a potential 200% plus discount. I suppose I could credit the account on the back end once I get reimbursed, but I would rather not have to worry about it.