Fixed cost projects and Actuals

Recently my company and i have been working with Zoho Projects. We are a consultancy which consults in microsoft. Now we use Zoho Projects for our projectmanagement and our invoice via books en zoho expense.

So the question is as follows,

We have two types of projects. We have Fixed price projects which we want to invoice by milestone. We also have a way of working which consists of monthly invoices which go aside the cost of the fixed cost project itself. Does someone know a way to do this both in the same project?