If you choose to promote pricey items to get more views or a quick sale, the costs can soon stack up.
Check your sold items in 'orders'.
Look at 'payment details' from the dropdown menu for individual items.
Check the 'Transaction Breakdown' for an explanation of fees. Fees charged for an optional promotion usually show as 'general ad fees' I think.
"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)