Silly question: how do I browse to the cart from localhost?

I don't understand your question.

I can't figure out the url to display the cart on the local dev tools. I can throw

-local

on the 3 ssp's but the cart url is just

<https://domain/cart>

if I try

<https://domain/cart-local.ssp>

I get a blank page.

Use https://domain/shopping-local.ssp#cart

Thank you @Martin

Slight modification: you will/may have to put the SSP application name, eg: https://domain.com/sca-dev-20192/shopping-local.ssp#cart