There is no such thing as a free postage label, unless you had a specific offer for same and the sale was within the specified timeframe. Offers are not open-ended.
What you almost assuredly did was list at $20 with free postage. That means postage is included in the item price, not that it is free for the seller.
If you used an eBay label, that would have been $10.60, plus selling fees, around $2.68. So $7 (or $6.72) would be right.
