Even better, say that you will give them 20 hours more for free (at a "value of X$"), and after that support will be charged at X$. Don't discount support, or y

Even better, say that you will give them 20 hours more for free (at a "value of X$"), and after that support will be charged at X$. Don't discount support, or you'll be working for months/years at that discounted rate. On the opposite side, you should charge more for support than for regular work.