Is purchasing anything unnecessary a sin?

Whenever we buy something there are many effects, some of which are not good. Some examples include supporting places of work that are rife with immodesty and are an unnecessary near occasion of sin to the people working there, companies supporting sinful behavior, companies doing unethical things, and the government using the tax to fund unjust wars. In these situations we must make use of the principle of double effect. One of the requirements for the action being permissible is that there be a *proportionately grave reason* to do it. I think it's fair to say that nothing other than sustenance and religious activities rises to this level. But since it doesn't, does that mean purchasing anything unnecessary is the mortal sin of scandal (*some word or deed that is itself evil* (funding evil stuff) *or has the appearance of evil* (not caring about funding evil stuff) *and provides an occasion of sin to another* (giving people money for doing evil things))?