Based on the details you provided, I recommend setting your blueprints up as inventory. This will allow you to enter them as an item or service, and then you can start charging your customers using invoices. I've included the steps below to show you how to add a service/item:
Navigate to the Lists tab and select Item List.
On the Item List window, choose Item then New.
Pick the type of Item you want to create and fill in the item fields.
(Optional) Press Custom Fields to add custom fields.
Now, you're ready to create an invoice for your customers. Here's how:
Go to the Customers menu and push Create Invoices.
From the Customer: Job drop-down, select a customer.
Fill in the Date, Invoice #, Bill to/Sold to, and Terms at the top of the form.
In the detail area, choose the items (blueprints) the customer is paying for.
Press Save & Close.
Here are a few articles you can check out for more information: