Credit cards
The facility is an unsecured loan with interest in the small print. One camp: signing it is haram, even if you never pay a penny of interest, because you consented to riba. Other camp: if you repay in full every month, use it only as a payment rail, and have a process that makes interest impossible, it may be tolerated for need (travel, deposits, building a file).
This guide does not pick a mufti for you. If you follow the stricter view, use a debit card, a true charge card that must be settled, or an Islamic card where the underlying is a ujrah (fee) facility without interest. If you follow the usage view, automate full repayment and do not keep a card “for emergencies” that then funds a holiday.
BNPL and late fees
Klarna-style pay-in-three is a sale on delay if there is no increment and no late-interest. The moment a missed instalment becomes a percentage, you are back in riba. Read the merchant’s lender, not the checkout button.
Time-based late fees on any bill — council tax excepted as a fine from the state — should be avoided. A flat admin fee for a real cost is a different argument than “1.5% per month”.