Backdated interest payments are leaving young people with unexpected bills from "buy now, pay later" deals, a charity has warned.
Citizens Advice said that thousands of people in their 20s who signed up to offers were struggling with debt.
These deals allow people to delay paying for items for an agreed period of time, such as six or 12 months.
Yet failing to pay in full by an agreed date brings interest charges backdated to the start of the agreement.
Missing a payment deadline could leave people with bills amounting to twice the original price, Citizens Advice warned.
The charity said this was a persistent issue, having helped 24,000 people with 50,000 catalogue or mail order debt problems in the past year.
These people had an average debt of £1,300, with young adults aged 25 to 29 most likely to have problems with catalogue debts.
One shopper bought a £700 laptop from a catalogue company on a one-year interest-free deal.
Citizens Advice said he developed health problems which meant he had to give up work, and he struggled to pay back the final instalment of £150 during the interest-free period.
He was left with an interest bill that cost nearly as much as the laptop.
The charity also helped a man who bought a £600 tablet computer on a similar one-year deal. He still owed £100 at the deadline.
As a result he received an interest charge amounting to more than the cost of the tablet.
Citizens Advice said its analysis of 250 cases revealed cases of high fees, inadequate affordability checks and poor debt collection methods, such as repeated demands for repayments from debt-ridden customers.
It is calling for the financial regulator to ensure providers display potential charges clearly, and make it clear that a failure to repay will lead to backdated interest payments and additional late payment fees.
"Buy Now, Pay Later deals help people spread the costs of catalogue purchases but it is vital customers understand what they are signing up for and what will happen if they fail to repay on time," said Citizens Advice chief executive Gillian Guy.
"Clearer explanations by catalogue firms when advertising these deals will prevent people being hit with shock bills that could send them spiralling into debt."
The Financial Conduct Authority (FCA) is conducting a review into high-cost, short-term credit, including catalogue debt.
"We will look across all high-cost products to build a full picture of how these are used, whether they cause detriment and, if so, to which consumers," an FCA spokesman said.
Any proposals will be published in the summer of 2017.http://www.bbc.co.uk/news/business-38378069