What is the ruling on prizes offered by bank card companies to users, and is purchasing fixed-interest certificates, such as 6%, considered usury (riba)?

Firstly: Dealing with usurious banks is not permissible except out of necessity, because it falls under cooperating in sin and aggression. Their offering of prizes to encourage dealing with them is a form of promoting what is forbidden. Secondly: Guaranteed profit is not permissible. If the government borrows from people and gives them a fixed percentage, these are forbidden usurious bonds, and dealing with them is absolutely impermissible.