A lot of companies make mistakes when it comes to charging their customers, and for anyone who has been in that situation can attest to, it isn’t always easy to set the record straight. A lot of people will end up just paying the extra amount because they can’t be bothered to go through the standard rigmarole which companies put people through when they protest at a charge.
The first step with any contested payment is to collect all of the contract details and any other information regarding your situation (delivery information or whatever else). You can then use that information to argue your case with the company, or if that doesn’t work, you can provide a governing body/watchdog with that information so that they can help you to sort the situation out. Often you’ll find that the introduction of a watchdog is enough to convince the company in question to decide in your favour, but if it isn’t then the watchdog will continue to help you sort it out.