Payments under $20 are settled in Naira rather than dollars, so you're charged the Naira equivalent instead.
The conversion uses the exchange rate at the moment you pay. The exact Naira figure is shown on the confirmation screen before you approve anything — if it doesn't look right, don't approve it. Contact support and quote the package number.
If the rate can't be fetched, the payment is stopped rather than guessed at, and you'll be asked to try again shortly.