Never used MyTyres myself but it's a massive operation and been around for several years, based in Germany like Tyreleader I think.
Looks like a good price for 20L delivered considering how much you might pay for just 4L.
Had a look on Ebay again and found this, http://www.ebay.co.uk/itm/20-L-LITE...831475?hash=item2c946e82f3:g:CwAAAOSw585WULEI
Works out at £28 for a 5L tin delivered. slightly more expensive than the single barrel.