How do I allow discounts on this form?

How do I allow discounts on this form? I would like to add a box where the buyer can self select for a discount. I tried going with multiple products, and a negative product value (for the discount), but it is disallowed. The use case is this:

product 1: $5

product 2: $10

self select discount for product 2: -$2

product 3: $15

self select discount for product 3: -$2

A buyer will only select 1 product (1, 2 or 3)

Thanks

This is a re-post of a comment on Order Form Types