In the US, it's still a line item on your receipt. Most places don't include sales tax on the shelf tag or price sticker due to complexity. You can have state, county/area, and/or city taxes that apply. In NYC, for instance, we have a 4% NY State sales tax, a 4.5% NYC sales tax, and a 0.375% NYC metro area sales tax, totaling 8.875%. Clothes aren't taxed in the metro area sales tax so are 8.5% unless they're under $110 and then they're exempt. Most food is exempt except prepared food. Then there's the issue of fractions of a cent as the sales tax is calculated on the total bill not each item individually. There's also the fact that taxes change now and then... they adjust what food it applies to or what the cutoff is for clothes, etc. Some organizations have a sales tax exemption certificate (if they are a business planning to resell for instance) and that must also be taken into account.

Due to complexity, all larger stores with multiple geographic locations would never have separate pricing signage for every single store, so they don't. Other stores do the same. It's all calculated at the register as things are scanned in the computer.

FYI, in the US, depending on the jurisdiction, some items are exempt from sales tax. E.g. necessities like food and clothing. In my jurisdiction, clothing is taxed by the county but not by the state.

Which is a long way of saying that even if you knew the total you spent at Amazon, you wouldn't be able to derive the total amount of sales tax paid.

> apparently in the US you have to do some mental gymnastics to manually add sales tax to the listed price to figure out how much to pay in the first place

FWIW, usually in the US it's just a line item on the receipt or checkout screen.