Coinbase said the charges were caused by Visa reversing and recharging transactions.

Over the last few months, banks and card issuers have asked the networks to change the MCC for digital currency purchases, Coinbase noted in another tweet, saying Visa changed the MCC for digital currency purchases to a code that enables banks and card issuers to charge customers additional fees. MCC is a system of categorization for merchants so tax agencies can follow the trail.

Coinbase claims it is working with major card networks to create a new MCC for digital currency purchases. The company said it hopes this will not include an additional cash advance fee.

Some buyers said they have been charged up to 50 times the intended purchases, according to thenextweb.