What you save, and what you actually pay.
The discount comes off first, then tax applies to the reduced price — the order stores actually use. Stacked promotions ("extra 20% off sale") multiply rather than add: 50% off then 20% off is 60% off total, not 70% — run the second discount on the first result to see it.
How do stacked discounts work? Sequentially: each percentage applies to the already-reduced price. 50% + extra 20% = pay 50% × 80% = 40% of original.
Is tax calculated before or after the discount? After, in most US states, for store discounts. Manufacturer coupons differ in some states — the receipt always shows which was used.