never done the massage thing but from what i gather, you pay the £20 or whatever for the massage, then around 3/4 to 2/5th the way through, they ask if you want any extras, then you get the money from your jeans, pay them and then there you go.
i don't think it's always offered though, just in the majority of cases. so if its nearing the end, then no harming in asking if theres any extras