PayPal (Europe) & KSP Rechtsanwalte

Hi everyone and thanks in advance for any help given. I just moved to Germany, Berlin and I am not so good with deutsch language so I will write in English to explain my situation better.

Today I received a letter from KSP Kanzlei that I owe PayPal 13.50€ and 43.24 € to them for various legal fees and that I have until 11.09.2018. to pay the invoice.
I made a purchase via PayPal on steam for 10 € and the amount was succesfuly charged from my account on 18.07.
What happened was they gave the 10€ back on the same day and on the 20.07. they made a new invoice of 13.50€ which was probably some sort of fees of which I know nothing about that was to take place on the 23.07.I at that moment didn't have enough funds on my account but I also didn't know that I owed anything because I didn't enter my PayPal account since and thought the 10€ bill was all it took.
I have 2 mails on PayPal because I made the account a long time ago and didnt use the original (@hotmail.com) account for some time so I added a new (@gmail.com) account but it seems i didnt place it as the formal email for PayPal emails.
Today when I received the letter from KSP Rechtsanwalte I managed to get into my @hotmail.com account to find a few emails from PayPal warning me that i have a -13.50€ status on my account.
I also found an email from KSP that i need to pay them also everything since PayPal passed the "Case" to them.
I had the funds on my account on several occasions but it didn't automatically take the required sum out of my bank account.
So basically I wasn't aware of any debt before the said letter came to my address and also it said that they had tried to contact me via Phone call but I never received any phone call of the sort.
Today after receiving the letter I went to PayPal site and payed the 13.50€ but I am not sure what to do with the warning letter.I read a lot of forum themes regarding KSP but none said what do do after paying PayPal sum.
Some say ignore it some say send a letter some say they usually don't go to court but I would like to be on the safe side in a way and also save 43€ for something i thought shouldn't even be.

Once more thanks in advance for any helpful advice.
Mit freundlichen Grüßen