Paid for car part during car service even though the car was still under service plan

I booked my car for a service in 2020 for 45 000km and they made me pay for the parts replaced even through my car was still under service plan . I sent an email to the part manager and he rep**** on the 6th of April 2021 saying they are investigating it internally. Since then I have sent follow up emails but he is not attending to it . It is very frustrating