If you are considering total costs Sherrard I would say quite a bit more. I'm not only taking into account the cost of fuel, but there's the wear and tear on tyres and mechanicals and increased insurance costs (the cost and hassle of any possible insurance claim in the event of an accident incurred during the shopping trip). It's hard to settle on a realistic figure though.
What you can safely say is that it's quite pricey to get your shopping whichever way you choose to get it to your home.