A couple of things....
1. In conditional formatting it won't accept SEARCH(OR("cash","check"),E18. You'd need to separate them into two separate SEARCHES with the OR in front.
2. Because you will be returning an error when the search finds nothing, that will screw up your OR statement too. You'll need to put your search inside an ISNUMBER.

Soooo, try
=NOT(OR(ISNUMBER(SEARCH("cash",$E18)),ISNUMBER(SEARCH("check",$E18))))
Does that work for you?

Or used DaddyLonglegs solution. Much nicer!