BNPL allows shoppers to spread the cost of purchases over weeks or months, using interest-free credit. But debts can mount if people miss payments.
From next year all BNPL apps will be regulated, leading to stricter affordability checks.
But in the meantime, debt advisors say people are using them "unsustainably" to "plug the gap" in their budgets.
Soft credit checks mean BNPL providers are often not told if people are borrowing from elsewhere - so they sanction loans without knowing a customer's wider situation.
Abi started using BNPL when she hit tough financial times.
"There's a temptation to go 'oh I'll just use that today and when I get paid, I'll pay it off' - and extend it over a few months," says Abi, who is training to become a barber.
"Then you have to go back and live on it and then do it again."
The 37-year-old, who cannot use credit cards because of other existing debts, began using BNPL three years ago to make larger purchases.