[dragonwriter]

> So as a hypothetical business owner in Washington who sells things online, I have to keep track of the tax laws of the other 49 states, and remit 50 checks quarterly (or maybe annually)?

Well, you have to either keep track of those laws for the states you choose to sell into, or use the free software provided by each state that has opted into the arrangement (the MFA provides an opt-in regime for states, with conditions including the provision of free software, with retailers not liable for any failure to collect and/or remit tax that is due to errors in the software.) But, yeah, you have to pay each taxing "state" (which will probably be less than 50 initially, but more than 50 in the limit case, since "states" under the MFA include the the 50 states, DC, plus other US territories and possessions.)

Or, instead of using the State-provided software and doing it yourself, you can pay a "certified software provider" to do it, and be immunized against liability for any errors that are the fault of that provider.