I will give you two examples:
#1 : You buy something that total to $19.99, and you don't want change. How are you going to pay the last 4 cents? or
#2: if you do want change, and you pay a 20 dollar bill, is the cashier going to say: "sorry, the penny got abolished, and so we don't have any pennies.
these are 2 ways customers would be affected at checkout lines if the penny is abolished.
hope this helps