Two discounts are not their sum
The single most common mistake with a sale price is adding the percentages together. An extra ten per cent off an item already reduced by twenty is not thirty per cent off. The second offer applies to the reduced price rather than to the original, so the two together take twenty-eight per cent off the label and leave you paying seventy-two per cent of it. Two lots of twenty, which sound like forty, come to thirty-six.
That is not a rounding quibble. On anything expensive the gap between what people expect and what they pay is the whole reason the second offer is worth advertising. This page shows the chain step by step for exactly that reason: each line applies to the price on the line above, which is the order a till applies them in, and the last line is what you hand over.
The single percentage the whole chain adds up to is printed beside the saving. That figure is what compares two offers with different shapes: a straight discount against a discount plus a coupon, or a large percentage on a higher list price against a small one on a lower.
Where the coupon and the tax sit in the order
A fixed amount off behaves differently from a percentage, and where it lands changes the answer. Here it comes after both percentages, which is the common case: a coupon knocks a flat sum off what you would otherwise have paid. If the shop applies it first instead, put the coupon in as the discount and work the percentages on what is left.
Sales tax is charged on the discounted price. That is how a sales tax works in the United States: the tax base is what changes hands, not what the label once said. It is worth noticing what follows from that, because it is the quiet second saving in any discount. Taking a fifth off the price takes very nearly a fifth off the tax as well, to within the rounding of a cent.
A value added tax is a different animal and this field is not one. Where prices are advertised with the tax already inside them, work out the discount on the price as shown and leave the tax field empty.
A coupon worth more than what is left does not pay you to take the goods away. The price stops at nothing and the page says so rather than printing a negative number.
What this page does not know
It does not know whether the list price was ever the real price. Several jurisdictions regulate that question, requiring a shop to state the lowest price charged in some period before the reduction was announced, and none of that is arithmetic. The calculation here starts from the number you give it.
It also has nothing to say about delivery, restocking charges, loyalty points, or offers that do not reduce to a percentage at all. Buy-one-get-one and three-for-two are discounts on a basket rather than on an item, and turning them into a single rate depends on what else is in the basket.