ParcelMonkey offer great prices, they use up free slots with city-link, average cost for a 50cmx50cmx50cm parcel up to 10kg is around £7 including vat and payable via paypal. User friendly website and shipping labels sent to your email for you to print - very convenient. Part of the order code is the tracking number to use on the city-link website, great so you can track it yourself or send to your buyer to keep good communication.
Myhermes offer simular value and easy use.
Still think Royal mail are cheaper for items under 2kg.
Happy selling :)