Sorry, but as far as I am aware, the only method by which the user is sent an invoice or receipt for any purchase is via email. I do not know of any other source that is user-accessible.
You imply in your post that you have not received an invoice for at least some of your purchases. This is definitely NOT my experience. Has this happened on more than one occasion? What was the software you purchased? Was it a recent or rather earlier purchase?
Theoretically, you should be able to email infoservice@magix.net and ask for all the invoices/receipts on your account to be sent to you but whether this is actually possible or practical I would rather doubt. (In my case I have been using Magix products for something like 20 years and I doubt I would be very popular at all with customer service if I requested all my invoices be sent to me, even if they still existed in my account!)
@Brian-Smith7539 Since Magix is located in Germany they must keep all invoices for at least ten years. So you should be able to get your invoices dating back to 2012 from customer service as @emmrecs already suggested.
Basically you could force them by European privacy law to hand you over all information they have saved about you. But I'm not sure if this is valid for customers outside of the European Union.