> Does anyone have any idea how this could affect any startups selling software online?

Directly, this law would have no effect on anyone except state policymakers if it passes.

What effects particular state policies electing to make use of the authority Congress is allowing states under this law would have depends on the exact policies adopted by individual states (which would affect what goods are covered by the states internet sales tax regime and what variables are needed to calculate the rates, and what, within the requirements of the law, form the free-of-charge tax calculation software supplied by the state takes.)