Account payable balance

Good day I bought a rain 5G aim with the box on a promotion then I paid 499 the lady promise me there is service by me when I got home that day no signal the whole day same thing over and over I send them numerous emails ,and they keep sending me messages it's been resolved and it was never resolves this think was showing over and over no signal then I cancelled it and asked for a refund and they send Ram to fetch their product but never refunded me now they send me a bill of 540 and I cant understand why so high when they never paid me and there 5G never worked everytime I send a mail it's always different people replying I will help you but never got help .there service is so bad and I never thought this would happened .