There is only one invoice for the total price.

Why are you asking the same question again after already receiving a reply here just two weeks ago?:

The answer is still the same whether you ask in English or with a German translation!

It’s not the same question: last time I was asking if the device invoice can be combined with the mobile phone bill. I was still expecting to get a separate invoice for the device though, and after the first instalment was deducted a few days ago, I realized there’s no invoice for that payment. This way I don’t have any document proving that I make payments every month.

It is not exactly the same question but you are effectively asking for the same thing - a monthly invoice. It’s therefore the same answer for the same reason. An invoice is not issued monthly because you are not being billed monthly for the device. You were billed once when you prurchsed it and are now paying it off in monthly instalments. If you need proof of payment each month, your bank statement would be a suitable document.

Good morning @igityan 
You have already received good support from bs0 
There are no monthly bills for an o2 my Handy purchase, but you have received the hardware bill and the installment plan. You can access the installment plan online at any time.
I hope that works for you.
Best regards 

Kathi