The rate from PJIA to Maho is $8. Thats for two people with 1 bag each. Add $2 each bag after that. Tip is whatever you feel comfortable with. Rates for taxis can be found here.
That rate chart appears to be from 2014. Not totally sure it would be up to date with current rates.
There is a dispatcher at the airport and they should have the current fares. To the OP, I would confirm with the dispatcher and driver as to the fare and tip 10-15%.
Agreed, I doubt that gypsy cabs would dare to operate at the airport. As far as a tip, if the fare is $12, $2 or $3 or $5, really isn't a big deal to me, in the scheme of things. They do charge a fee per bag on top, so figure $1 per bag, plus 20%, which on $12 is under $3, not major money regardless..