Ruling on buying a phone or car on instalments if the contract says that if I pay late, I must pay either (1) interest on the overdue amount, e.g. 3%, or (2) a fixed late-payment fee, e.g. $25. Is the ruling the same for contracts like cellular data plans or car lease? I intend to pay on time.

Ustadh AbdulAziz al-Haqqan

A penalty for paying late is riba whether it is a percentage or a fixed fee. If the thing can be had outright or on a plan without that clause then take that, and if it is a service you cannot do without and every contract carries the clause, there is no blame on you.