It's 200 customers in SD or $100k in sales. If you scale by population, for California, it's about 9100 customers and $4.5 million in sales. Don't expect California to scale it.
As others have pointed out, it's a major hassle for retailers if this gets imposed nationally. 45 states have sales tax. So there are at least 45 state returns that have to be filed each quarter (or possibly more often), opening you to audits, business license requirements, state income taxes, etc.
Each state, and sometimes municipalities and other taxing entities in states have differing rules on what is taxed. Example, in Texas food isn't taxed if not for immediate consumption, so most grocery items aren't taxed, but a Happy Meal is. For quite a while, a state ruling was that 6 donuts was "not for immediate consumption". So a small pack of 6 Hostess mini-donuts was sales tax exempt, but two Twinkies were not.
You need to have detailed geodata information on what is and isn't taxed in an area and what the rates are there. For a big company like Amazon, it's not a significant expense. But a small diversified business is screwed.
Congress needs to get on this quickly with some national guidance. At a minimum they need to have one form to fill out for sales taxes all over the US. And having one place to file it would be nice.