It genuinely doesn't happen. Can you imagine thousands of sellers overlooking such a big error on a daily basis?
If you sell an item for £20, plus £4 postage, you would receive £20.
Look in 'order details' for a breakdown of what the buyer paid and deductions.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)