If you are in Greensboro, NC, you will not be able to get on one of those flights. A US passport will still keep you out, unless you spend the 14 days immediately prior to your flight to Costa Rica in, say, Canada. You can't fly North Carolina-Canada-Costa Rica with a simple connection in Canada.
Visitors also need to present a negative COVID-19 test that was taken no more than 48 hours before arrival and purchase insurance upon arrival in Costa Rica that would cover any possible medical expenses.