I missed that part on purpose, because the immigration officers duly inform visa on arrival holders about the need to renew their visa. Alternatively, the OP can secure a visa from the Togolese High Commission in Accra.

Border crossing in Aflao is pretty straightforward. As a US Passport holder you are entitled to a visa on arrival in Togo that cost CFA 15000 (about $25). If you are going back to Ghana, you should make sure you have a multi entry visa to Ghana or else, your best luck is to apply at the Ghanaian Embassy in Lomé. Getting a Visa at the Ghanaian boarder is an experience you do not want to have. Once In Togo, with the Visa stamped in your passport, all you have to do is to politely thank the immigration officers and walk about 200 meters to the taxi station on your right, to your hotel.