We want to create coupons that offer a percentage discount only for the product(s) selected by the customer, but not for the entire transaction, which also includes a variant. For example, a customer might select three products priced at $50 each ($150), and three variants, priced at $25 each ($75), and a handling fee of $20 is added, for a total order of $245). We would like the discount to apply only to the three products ($150), and not the variants or handling fee. How can we accomplish this? Thanks.