Rather than becoming complicated it much better to assume discounts are already complex. Rebates (final bill - X$) aren’t the same as exclusive discounts (X% for seniors or Y% for veterans) which are different from additive discounts ( X% + Y% = Z%) as separate from multiplicative ( (1 - X%) * (1 - Y%) = (1 - Z%)) etc.

Unfortunately, it’s really easy to assume everything is simple until the system gets complicated enough nobody understands it.