Just looked at my credit card statement. The departure tax at Liberia Airport was $28.58 when I left Dec 9. You are directed to a window to pay this, where, conveniently, they accept Credit Cards. BE AWARE this is not a credit card transaction.... it is processed as a CASH ADVANCE....which means, as I just discovered, my credit card charged me $10 plus interest!!! I am a pretty observant and alert traveller and was in no rush. I did not see any sign indicating this or I would have handed them the money in US cash!
I hope someone learns from my experience!