If you need a proof of payment you can print it yourself, it is quick and easy to do.
Log in at this page http://www.quant-investing.com/login
If you have forgotten your username click the link Forgot your password? below the Login button
After you have logged in click the new menu item Account which will appear in the top menu
On the next page click the button Maintain subscriptions
Under the heading Payment History you can see all past payments and print out the details.
If you need a more detailed invoice than that please send me the following details:
Your full name or company name
Full physical address
VAT Number if applicable.