If you have a 3% gross margin and a million a year in revenue, you don't have enough money to hire even a single full time employee. Who is going to handle the new requirement to integrate 50 different software packages this bill requires the states make available to compute sales tax at purchase time, to fill out 50 sales tax forms every quarter/year, to cut and keep track of 50 checks...

A $1M/year seller could easily spend their entire year's profit just paying someone to keep track of which of the states have opted in to the new programs this bill will create, research the sales tax laws and forms in those states, research their software/APIs, build the integrations, etc.

If a federal sales tax law simply meant cutting one check and sending one spreadsheet to some authority that then divvies out the payments to the appropriate governments, that'd be something else. I don't think that's been proposed.