Buy your ticket online and if they ask you at the counter at check-in do you have a return flight, have a date and airline name (not Jet Blue) ready...
Example:
Good Morning,Miss------, and where are you flying today?
Hi! I'm going to Santo Domingo
You do know that you are required to have a round trip ticket to travel to the DR?
Yes, M'aam, I'm leaving on December 2nd, but I'm returning on American Airlines
Oh, too bad...well, have a nice trip. Your gate is Gate 21.
Thank you
I am telling you this because I have been asked before but if you answer smoothly without hesitation, they do not ask for proof.
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SHALENA