If you're voting by mail in Missouri your ballot affidavit MUST be notarized
If you fail to get your affidavit (on the back of your ballot envelope) notarized, your ballot WILL be rejected. You will not be notified, you do not get the chance to vote again. Your ballot has already been "cast" but will not count.
Keep in mind that notary's CANNOT charge a fee to notarize election mail. ONLY sign your name on the affidavit once you are in front of the notary public.
The ONLY circumstances under which the notary requirement is waived is if you are voting absentee by mail due to "Incapacity or confinement due to illness or disability", if you are voting from an active duty military position, or a citizen living overseas.