If you have already given them a quote then you should try and stick to it. Ideally you would add your supplies to your labor costs to create a quote to give to your client before you start painting. The only reason you would change your quote after you have given them the quote is if they wanted you to use a more expensive paint or do more painting.

two ways to bid 1)time and materials 2)flat rate you discuss both options with your customer they see your trying to save them money you get callbacks its good buisiness