Get overcharged and then unwilling to send p.o.p

I had been overcharged regarding my payment due to a tracking system not listed as it was installed in the same week i recived my car. I have requested that you send the proof of payment that i can see you have refunded me as i do not see anything on the date provided. I am not impressed as you have slacked in this i have been asking for the proof over a month and you are not worried about the service provided to me as paying customer. I honestly dont understand why you are being so against sending me the proof of payment. As customer i have spoken and mailed you regarding this i hope you wil assist me now and supply the service you would expect from a company your size. I honestly think it is very poor service ro only tell the paying customer check on this date as i have been making my payments to you the least you can do is send the proof of payment not give a poor service after all it was a mistake made by you