Royal Mail has admitted that ongoing errors with its website has led to some customers being double-charged for postal services.
The problems on Royal Mail's postage websites for consumers and businesses, including a Price Finder page, emerged last week.
They were revealed to have been caused by a migration of online data to new servers, which knocked some postage applications offline since 21 November.
Around 600 customers, out of a total 30,000, have been affected by the problems so far, with users resubmitting payments after being told that their initial payments had failed.
In fact they were charged twice, at an average of £50 each time. The maximum incorrect charge was £200.
Royal Mail and Capgemini, which manages the website, have apologised to customers for the disruptions to the services in one of the busiest periods for the postage company.
"We would like to apologise for the difficulties some Royal Mail customers are experiencing with a number of the applications on the website.
"We very much appreciate that this is the busiest trading period of the year for many of our customers," said Moya Greene, chief executive of Royal Mail Group and Paul Hermelin, CEO of Capgemini, in a joint statement.
Other problems include some customers not being able to print out postage they have paid for, or not being able to see top-ups on their pre-pay account.
Royal Mail added that there have been issues with a "small" number of services on the Parcelforce Worldwide and Post Office's websites, but said that these sites were "largely unaffected".
Customers who have been charged twice by the system will receive a refund and a "goodwill" payment of £25.
The problems with the SmartStamp and Online Postage applications are ongoing, and Royal Mail said: "Capgemini is working hard to return the website to normal."