For starters, take a look at this demo file for creating and printing Invoices: http://fmforums.com/forum/showpost.php?post/309136/
Each product or service you deliver can be a record in the Products table with an assigned price. You can also leave the price blank and fill it in manually when you add it to your invoice.
One record in your products table could be "base price" with your $25.00 fee. The next record could be "1 level window" with $2.00, A third can be "2 level" with $2.50 as the price. You can also add a trip fee with a price based on one mile of travel or you can leave the fee amount blank or you can add a field to your invoice table for the trip fee.
So if you have 2 level house, with 5 ground floor windows and 6 second floor windows, you might add 3 records to the portal. The number of windows would be your quantity.