Is paying for purchases in installments for an extra monthly charge, or deferring payment for an increase, considered usury?

It is not permissible to buy from the intermediary website in this manner due to the presence of usury. The website pays the money on your behalf then recovers it with an increase, and this is a forbidden usurious loan. It is not a condition for a usurious loan that the lender pays the money directly to the borrower; rather, it suffices that he pays it on his behalf, and every stipulated increase in the loan is usury. Al-Qurtubi said: "Muslims have unanimously agreed, narrated from their Prophet, peace and blessings be upon him, that stipulating an increase in a loan is usury, even if it were a handful of fodder - as Ibn Mas'ud said - or a single grain." The permissible alternative is for the website to buy the commodity for itself and take possession of it, then sell it to you in installments or on credit, and this is called Murabaha.