One possible reason that no one's mentioned... It isn't about the money you pay, but the money you keep. If I buy a few things for £4.99 then without a bag I

One possible reason that no one's mentioned... It isn't about the money you pay, but the money you keep. If I buy a few things for £4.99 then without a bag I can pay with a £10 note and get a £5 note back, but with the 5p charge I receive loads of coins in change. Plus the cognitive pain of monetary amounts like £5.04