Are you buying it from the company that are fitting it?
If so then tell them 2 do it for free, or you will go elsewhere! It only takes about 10-20 mins tops to change a back box! Im getting a full new standard exhaust fitted 2moro and they are doing that for free, also gave me a good discount on the exhaust as they said they would match any other local price. £85 for a full system!