To have the invoice click on your store name on the top right corner of the Stock Sync page and click Manage Billings.

Click on the Manage Payment info link:

Then a link from will pop up. You will need to scroll down until you see the Billing History as per the below screenshot:

Once clicked on the button, it will bring you to a page where you can download the invoice:

Please give it a try and do share your thoughts or ask us any questions if you may as we intend to make your life easier with our applications.