I’m paying for 100mbs only getting 10mbs

I recently upgraded my internet only getting 10 to 20 mbs instead of the 100. Talked  to customer care they said something about a weak signal. So a technician should come to my house within a couple a days to check it out. Everything sounds fine until the advised me I might have to pay 75 dollars. Why do I have to pay for something that’s not my fault?