Dear @Pingu99 ,
first of all, welcome to our forum. Nice you have found your way to us. :-)
I took a look at your invoice and will explain it to you.
You ordered two contracts.
One is a o2 Free L Boost with 60 GB data volume a month. You’ll have to pay 44,99 Euros -15 Euros (young people doscount) = 29,99 Euros a month for it (because of the billing period you have credited just one day in this invoice).
In the first invoice you also have to pay the one time connection fee, which amount to 39,99 Euro. The fee arises for new contracts.
The onther one is the o2 My Data Spot Unlimited Kombi. It belongs to the o2 My Home Spot you bought. It costs 39,99 Euros - 20 Euro (2nd SIM discount) - 5 Euros (hardware discount) = 14,99 Euros a month (because of the billing period you have credited just one day in this invoice).
In the first invoice you also have to pay the one time connection fee, which amount to 39,99 Euro. The fee arises for new contracts.
one day of 29,99 Euros costs 0,97 Euros
one day of 14,99 Euros costs 0,48 Euros
+ 39,99 Euros
+ 39,99 Euros
----------------------------------------------------------------------------
81, 43 Euros
So next month your invioce for the two contracts amount 29,99 Euros +14,99 Euros = 44,98 Euros.
I hope I could bring a little light into the darkness. 
Do you have any questions about using our community? Then take a look at Neu in der o2 Community? Die wichtigsten Funktionen und Hinweise auf einen Blick. There you will find all the information you need to use!
Kind regards,
Andrea