The Co-op will refund hundreds of thousands of shoppers across the country following a technical glitch on July 7 that saw them charged twice.
Customers were charged double the amount for their purchases when using a credit or debit card.
The company helpline has gone into meltdown as it tries to handle the high volume of calls, and has urged people to be patient.
Co-op has stated that refunds will be paid back into the account used to make the purchase.
"Due to a processing error, customers who shopped with us or used a petrol filling station on 7 July using a credit or debit card were charged twice," a spokesman said.
"We would like to apologise to all those affected, and reassure customers that refunds will be made directly in to their accounts within 24 hours.
"We will also reimburse any customers who have incurred bank charges as a result of this error. These customers should contact our customer careline."
Those who have been received these double charges can expect to be refunded by July 10.
Customers can contact the company's helpline on 0800 068 6727, although when IBTimes UK tried, there was an automated response saying the company was experiencing "telephonic issues".
"This is largely understandable due to the high volume of calls."