Having difficulties with submitting additional information to a application already made

Hey Charles, I had the same situation — my nationality is also USA, but my birth country is Cameroon. I recently learned that, for applying for an online eVisa, the Cameroon government now requires a legalized invitation letter from family. This invitation must be obtained from the local police station in Cameroon, include a fiscal stamp, and outline the details of your trip. I contacted my family to arrange it, and with that document, I successfully received my eVisa a few days ago. I hope this information helps you too.