Slow poor service

In June this year I upgraded to a Samsung Galaxy S8. In October, the charging started acting faulty, it would not charge properly, and the fast charging function stopped working. I sent the phone back for an assessment and repairs. They sent me a bill for fixing the phone, but with no explanation as to what is happening and why I must pay, is the phone not under warranty for manufacturer faults, as this was not caused by me or by negligence on my part? I asked for a copy of my contract agreement from when I upgraded so I can see the warranty clause to see why I must pay. 2 days later no reply, so I send another mail complaining, a day later they reply saying the query has been forwarded. 5 days later now, no word, no contract, no warranty, etc...... Im done emailing, thats why Im here.