What to do when claiming reminder costs?

Questioner

Dear, On January 17th I got a call from my dentist's assistant that there was still an outstanding bill of 11 euros. I didn't know anything about it and paid it right away. It appears that the company Famed does the administration and had sent the invoice by email and a second reminder. I probably clicked this away without reading it because I receive so many unauthorised emails for payments. Apparently the term had already passed by 1 or 2 days. Today, January 31, I was called by famed and was told that I still have to pay reminder costs. 40 euros over an amount of 11 euros otherwise it will go to a collection agency Please give your opinion and what to do. Please respond

Lawyer

On 17 01 the invoice was paid to an authorized recipient, the dentist could have informed his contractor and 40 euros on 11, ridiculous.