over charge

I Ordered on the ubereats app and had a refund credit which i app**** to order , when the driver delivered his name was Tinashe he showed me the kfc slip and insisted i pay the full amount as uber did not apply my credit on , i checked the app and it also showed i owed the full amount so i paid him cash full. 3 weeks later when i wanted to place another order the credit was not there , when i checked on my last bill they showed that they added the credit on that obviously after it was delivered. i paid driver full so i sent several messages querying this as i have overpaid them for my order but they do not respond and there is no contact number to call them . This is really wrong as they literally stole .