Did you use a discount code at checkout? If yes, it may initially appear like the wrong item was credited however we deposit the final amount paid after the code was applied. To check this refer to the invoice sent with your order or log in to My Account, select 'My Orders' and check the item amount paid.
Also, please check your invoice to make sure the item you sent back was a Showpo item, and not something from a different retailer (it may sound silly, but it does happen).
Otherwise contact our Customer Happiness team via live chat, or email below and we'll happily sort this out for you!