Actually the shoe example can work pretty well when we incorporate the elements at play with the progressive tax system and the way that your tax is calculated as one lump sum during tax assessment.
In this case it would be the inverse in that more shoes are cheaper as opposed to more money means higher taxes....but the logic is the same.
You want to buy a pair of shoes... they cost $140..... There is a progressive discount being offered by the retailer. If you buy a second pair you will only pay $100 for it. The shoes are identical in every way as the discount is on two or more of the same item. You buy two pairs of shoes for $240 online and delivered to your doorstep.
You go for a jog in the park and you mate says...spiffy shoes.... how much did you pay for that pair of shoes? Go.
(20min delay)