The mail strike could lead to large numbers of credit card holders being hit with penalties as cheques are delayed in the post and payment deadlines missed as a result.
Customers who don't pay on time could end up being charged around £12.
Richard Brown, chief executive of price- comparison site Moneynet, warned: "With few or no deliveries last week and a huge backlog of post to be dealt with, millions of people could be affected unless they pick up the phone to their lender and stamp out the possibility of being treated as a bad payer."
Barclaycard, Britain's biggest credit card firm, with nearly 10 million customers, said it would take a "sympathetic" line with those whose cheques are stuck in the post.
"All we ask is that customers get in touch if they think there is a problem. We can then discuss alternative payment methods with them," said a Barclaycard spokesman.
It is not just credit card customers who could find themselves in the firing line. Some utility companies may also impose charges for late payment, and other gas and electricity customers may miss out on an early- payment discount.