Is obtaining a credit card—which involves a usurious interest condition for late payment—considered as sinful as explicit usury, which is among the major sins that exclude one from Allah's mercy, even if the card is not used or is used only within the grace period?

A Muslim is not permitted to enter into a contract that includes a forbidden condition, such as usury (riba), unless there is a necessity or a considerable need, because signing it is an affirmation of a forbidden act. Writing down usury is not the same as consuming it, but the one who writes, the one who consumes, the one who authorizes, and the one who witnesses are all equal in being cursed. Therefore, one must avoid this unless there is a considerable need, and one should intend to repay on time to avoid usury.