You don't need a comma before 'because.' In the case, use 'that' instead of 'which' because the only clothes you will buy are ones that fit. In other words, the clothes have to fit. If it were optional, then you could use 'which'; however, it isn't. I hope that explanation makes sense. I tend to use store instead of shop. Shop is not wrong, just store sounds more natural not me. Good job!