You may refer to:
Hotel Nyay Mandir vs Ishwar Lal Jinabhai Desai (RP No. 550 of 2006, decided on December 14, 2010)
The District Consumer Forum in Bharuch, Gujarat, asked a hotel, which charged more than the MRP on four bottles of a soft drink, to refund the excess amount of Rs. 22 collected from the consumer. In addition, it awarded a compensation of Rs. 5,000 and costs of Rs. 1,000 to the consumer and also directed the hotel to deposit Rs. 1,50,000 into the Consumer Welfare Fund. This was upheld by the consumer courts at the state and the national levels.
You may go thru her enlighting and educative article at:
https://www.hindustantimes.com/News-Feed/ColumnsOthers/Court-can-help-if-charged-more-than-MRP/Article1-854922.aspx
This article is THE solution for your issue.
Keep the original bill, bottle of the beverage with MRP on it, and payment receipt safely.
The concerned central Govt. Ministry is at:
https://fcamin.nic.in/
Dept of Consumer Affairs at:
https://consumeraffairs.nic.in/consumer/index.php
Consumer Info at:
https://consumeraffairs.nic.in/consumer/?q=node/4
NATIONAL CONSUMER POLICY at:
https://consumeraffairs.nic.in/consumer/?q=node/287
Complaint can be lodged under:
Legal Metrology
The details of it are at:
https://consumeraffairs.nic.in/consumer/?q=node/114
and download PDF file.
Complaint can also be filed under Consumer Protection Act.
You may log on to your legal metrology website of your State Govt and lodge complaint and demand that previous min. 3 years bills of the company be examined to determine the amounts overcharged from customers.
You can file complaint under the Consumer Protection Act also.