An invoice is a commercial instrument indicating payment 'due' against goods purchased. Invoice indicating prices, date of shipment, delivery and payment terms. So creation of this document (i.e. : Purchase Invoice) easily and keep the records of every document using this section.

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

Basic Detail Section :

Supplier Name : Please select supplier name.
Address : Fill address details.

Purchase Invoice No : It automatically generated when we create any new purchase 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 purchase invoice.
Currency : Please select currency.
Add Items : After addition of basic details, click to 'Add Item' to add in purchase invoice.

Item Details :

Item : Select an item.
Taxation : Select tax to apply on item. 
Quantity : Mention quantity.
Shipping Charge : Mention it ,if money charged on shipment of receiving of goods.  

After successfully insertion of record, We got Purchase 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 purchase 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 paid 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 supplier who is related this document.
   Note : Please select 'From Email' , using which email want to send email.
   Paid : For pay money against this invoice, and updation in accounts books.
   Cancel : For cancellation of invoice.
5. On Paid :
   [ Send, Manage Attachments, Print Document ]
6. On Canceled :
   [ Redraft, Manage Attachments ]
   

After successfully creation of Purchase Invoice, Now you have to further move to 'Insert record of Discount Voucher' for giving discount to your customer at online "E-Commerce  web'" using these vouchers during online purchasing.