What is the best fax app for MacBook Air

Apple removed the ability to connect a Mac to a modem quite a while ago, so you won't be able to fax directly from your Mac. You may be able to use a multifunction printer that has fax capability, if the printer manufacturer provides software that will allow that, or you can use an online service that provides a fax capability. RingCentral Fax is one I see mentioned, though I have no experience with it so I can't comment on how well it works.

Regards.

For very occasional use, going to a place like Staples, the UPS Store, or a FedEx store, all of whom often offer fax services, is indeed probably your most cost-effective solution, though of course not as convenient.

Regards.