The picture in the article shows a terminal whose percentages are clearly calculated post-tax. If you're the type that traditionally tipped percentages based on

The picture in the article shows a terminal whose percentages are clearly calculated post-tax. If you're the type that traditionally tipped percentages based on pre-tax bills, this is also something to look out for. A 20% pre-tax tip could very well come in under their lowest recommended option of 18% post-tax.