Also if you type in "cus" and it defaults to "customer" the original word "cus" should still show up in the list on the far left. If you click it it will move on and not replace it.
Not trying to step on the previous posters toes but to clarify, on the bottom of your text/post/email you have the word suggestions. Simply press the first one which is the one you've written, in this case cus and that will solve your problem.