[jeanvalmarc]

Amazon just lists it as a "subtotal," which I think is probably the best way to do it. I don't know about the "UX designer in a couple of hours" line: automatic shipping is a nightmarish bag of worms and isn't really a UX problem. What do you do if they order multiple SKU's that don't pack nicely into one box or are warehoused in different locations? Or if there is something weird about their shipping address? What do you use for box size and weight, and what approximation did you use for dunnage?

You can approximate UPS/FedEX costs by fitting a trend line, they are decently modeled by a linear (base charge + K*distance) function, but when you go to buy the label you might be way off. This puts you in the lose-lose-lose of either eating the difference of wrong estimates, overcharging for shipping and losing conversions, or just making people hate you by increasing the shipping over the estimate. Making a shipping API call is noticeably slow, so most people require an interaction after the address is entered.

Tim Sweeney's hot take was that "the two hardest problems in computer science are cache invalidation and shopping carts!"