I used
www.vacacionartravel.com to book a NAS-HAV flight; all the US agencies (and many foreign ones) seem to slam on a bunch of extra fees for example with Vacacionar my ticket total was $313 but with Havanatur, Nash and others it's quoted around $350.
I also inquired with a Canadian site Wowcuba but their credit card fee was about $20 higher, they also accept US cheques and money orders but even then they tack on a $15 fee for that (on top of a $35 'booking fee' that was already in the initial quote pkus it's kind of a hassle to mail and all. Crazy how they get away with these frivolous fees but they were still cheaper than the US geared sites.
I know nothing about Nash but was reading the Lonely Planet forum and a lot of negative comments... having a US based credit card is usually not a problem on any site they won't process the transaction as 'cuba travel'.