You must be using Simple Postage. There is no such postage cost using this new system, but as you have 87p on your listing that is what buyer pays.
I assume what you are selling falls into a category that avoids using this not so simple postage, so you print nothing produced by ebay, just address and stick an 87p stamp on.
If you have printed an ebay label, you should cancel it, as on low value items costs can exceed value if item.
