I would recommend that you set out very clearly at the beginning what you are providing and at what cost. You should also agree at that stage with the customer what can be flexible (i.e it doesn't cost you any money to give them extra) and what can't (and how much you will charge for 'extras'). This way your customer knows what he's getting for his money and what he's have to pay for as extra, should he change his mind later. At the end of the day, you want your customers to be happy with what you do for them (and come back again and recommend others) but it can't be at the expense of your profit and future of your business. In short; agree it up front. You should be able to build some flexibility into your charging structure. Good luck!