Trade and Commerce
A State Legislature may: (a) tax goods imported from other states/UTs the same way it taxes similar locally-made goods (without discriminating in favour of local goods); and (b) impose reasonable public-interest trade restrictions within that state - though a Bill for (b) needs the President's prior sanction before it can even be introduced.
This lets states impose their own reasonable trade-related taxes and restrictions, but with a crucial non-discrimination safeguard: a state cannot tax goods coming from other states more heavily than it taxes its own locally-produced equivalent goods - preventing states from using taxation to protectionist advantage for local industry over goods from sister states.
A state can charge a local entry tax on goods coming from another state, but only if it charges an equivalent tax on similar goods made within its own borders too - it cannot tax "outside" goods more heavily just to protect local producers from competition.