When making a purchase, you will get an email with the purchase order. 

Sometime the email ends up in spam box, so it can be a good idea to check what is in there.

We are handling all of our transaction from the USA using FastSpring online shopping professional service.

For FastSpring, you can get the invoice using https://sites.fastspring.com/tsplus/order/invoice/JWT12345-5638-XXXXX Where JWT12345-5638-XXXXX is your Purchase Order number.