Unless you have VCDS or equivalent to recode the battery details into the correct blocks you will have all sorts of problems throw up on the dash, RING an independent garage and get them to quote to fit it for you, I wouldn't use either of the 2 tyre centres you have had quotes from nor Halfords, as none of them know how to correctly code the battery in, and as Gareth has said, a garage will supply and fit what they get from the local motor factors, unless of course you ask for a certain battery, also garages like shops have to mark up their prices to RRP to make a living, before we had the internet you would only get garage prices, unless you knew where to go to get a better deal, now you can get prices less than RRP online