For every regular order, the payment platforms like SWREG or RegNow will send a confirmation email with the link of the invoice included. You're able to find the electronic invoice from the web page below.
http://www.mycommerce.com/order-search
The 2Checkout payment platform will send three confirmation emails. One is about product info, one is about the order info and the other is about payment receipt. In the payment receipt email you will find the receipt entrance.