MukaPuce68, Welcome to the forums!
Did you talk to Adobe Customer Care? They can pull up your records - I dont think you should send a copy of your bill. If you have an active order against a registered e-mail id or a valid order number, they should help you process a serial number request.
If you have registered your product with Adobe previously, you will be able to find your serial number after you log into your account. Look for the serial number in the My Accounts section.
If you have not registered your product, get in touch with Adobe Support. You can find more information in this article http://helpx.adobe.com/x-productkb/global/my-serial-number-doesnt-work.html.
This document has a chat link using which you can directly get in touch with support. Do get back in case if you still need more help.