devizo

There is no suffix in this word. Esperanto has no -iz- to find, and dev- (must, from devi) has nothing to do with it: deviz/o is a single root borrowed from French devise, and a beginner who cuts it into must-something has been misled by a shape.

French devise carries two senses, a motto and a foreign currency, and Esperanto imported the pair rather than choosing. The link between them is older than either. Old French deviser meant to divide or distinguish, from Latin dividere, and a heraldic device was the mark divided off on a shield to say whose shield it was. The mark acquired words and became a motto; the mark on a bill of exchange became the money. English device and devise came down the same road from the same word.

In practice the motto sense is the one you will meet: on coats of arms, on school gates, at the top of manifestos. The currency sense lives almost entirely in banking prose, usually in the plural, devizoj.