So ...

In order to create and send a new invoice to your customer, simply go to WP-Invoice => Add New.

You will be asked to provide the client’s email address:

After clicking the "Create New" button, the invoice editor screen will be loaded.

Is a regular editor page with the usual title field and the good old visual and text editor.

Just below the post editor box, you will notice a metabox where you can add your items.

Simply enter the item, the quantity and the price.

Also, you can click the toggle description link to add a description for an item included in the list.

Additionally, you can also click on the "Add Discount" button to deduct a discounted amount based on percentage or a fixed amount.

Here's the metabox:

And below is the the payments gateway box showing your default payment and all the payment methods you have enabled on your site. And of course, you can change these settings for a particular invoice if you want:

Also, on your right hand, you will see the user information metabox.

It will be showing the client’s email address and you can fill in rest of the information.

In case you don’t have all the information, you can leave it blank and the client can fill it in when making the payment.

And finally, in the publish metabox, there are options to publish this invoice as a quote, allow partial payments, or make it a recurring bill.

The "Toggle Advanced" link will allow you to add a due date for the invoice and to enter additional tax information:

OK. Time to see the real magic ...