To the best of my knowledge there is no direct bus service from the airport to Heredia.
In Costa Rica almost all buses originate in San Jose. The Alajuela bus does stop at the airport before it goes to Alajuela. You could most likely take it to San Jose and then catch the bus to Heredia. The good news is that the Heredia bus stop is right across the street from the Alajeula bus terminal in San Jose.
Just make sure the bus at the airport is headed to San Jose not Alajuela unlike US buses they do not display the destination on the front of the bus.
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Pura Vida

Only Irish coffee provides in a single glass all four
essential food groups:
alcohol, caffeine, sugar and fat.
Alex Levine
