Legislation for electronic payment systems (including credit cards) which charge any seller fees, to force these payment systems to round the original sale price down to the nearest dime amount (tenth of a dollar):
A seller selling a widget for $3.99 would automatically “lose” 9 cents if the customer pays with a credit card, because the credit provider would by law interpret that as $3.9, incentivising the shop owner to change the price either to $3.9 or $4. This would help consumers understand products' real costs with fewer and weaker math skills, and also decrease the number of pennies which need to be kept in circulation.