Sales invoice is a commercial instrument indicating payment 'due' against goods sold. Invoice indicating prices, date of shipment, delivery and payment terms. It is prepared by seller to his buyer. So creation of these documents easily and keep the records of every document using this section.

Click on Commerce > Sales Invoice Menu :
You will get page:

 

Basic Detail Section :

Customer Name : Please select customer name.
Address : Fill address details.

____Billing____
Will be used for organization / company to collect the money after delivery of products.

___Shipping___
Will be used for organization / company to dispatch the delivery of products.

Sales Invoice No : It automatically generated when we create any new sales invoice.
Create Date : Mention creation date, by default it takes present day's date.
Due Date : Expected date for receiving of money against this invoice.
Terms & Condition : Please select terms & condition for particular sales invoice.
Currency : Please select currency.
Add Item : After addition of basic details, click to 'Add Item' to add in sales invoice.

Item Details :

Item : Select an item.
Taxation : Select tax to apply on item. 
Quantity : Mention quantity.
Shipping Charge : Not required to mention, if it is predefined in that particular item.

After successfully insertion of record, We got Sale Invoice detail here.

Actions And Their significant :
On almost every grid in epan, you will see Status (Active/InActive or Draft etc.) and action dropdown. 
In sales invoice there are multiple kinds of actions based on status :

1.  On Draft :
   [ Submit, Manage Attachments ]
   Submit : For submitting document for approval by higher authority.
   Manage Attachments : For manage new attachments with this document.
2. On Submitted :
   [ Approve, Redesign, Manage Attachments, Print Document ]
   Redesign : For redesign this document.
   Approve : For approve this document, invoice approval means its being due for receive money against this invoice.
   Print Document : For print doc.
3. Redesign :
   [ Submit, Manage Attachments ]
4. On Due :
   [  Redesign, Paid, Send, Cancel, Manage Attachments, Print Document ]
   Send : For send this document to customer who is related this document.
   Note : Please select 'From Email' , using which email want to send email.
   Paid : For receive the money against this invoice, and updation in accounts books.
   Cancel : For cancellation of invoice.
5. On Paid :
   [ Send, Cancel, Complete, Manage Attachments, Print Document ]
6. On Canceled :
   [ Redraft, Manage Attachments ]
   

After successfully creation of Sales Invoice, Now you have to further move to 'Create a new Purchase Order' to handle the purchase records.