if the shopkeeper is taking more than that shows in the price tag , you can compalint to the authorities . its a punishable offense , according to qatar law . so you have the right to lodge  a complaint .

you can file the complaint online by providing detailed information including the shops name place etc

just follow this link to file a complaint

http://www.baladiya.gov.qa/portal/page?_pageid=253,344990&_dad=portal&_s...

good luck