US VWP: can I fly into the US and return home from Costa Rica?

I am planning to go to the US, entering under the Visa Waiver Program. I fly into Los Angeles on October 22nd and, 2 months later, go to Mexico. I'll go by land to Costa Rica and, from there, fly to my home country on January 16th.
I've read online that proof of onward travel to Mexico isn't sufficient when leaving the US.
Is having a flight out of Costa Rica sufficient? Would I also have to produce evidence of getting to Costa Rica?
I was hoping to travel by land, so I will not have any tickets with me except for my return flight from Costa Rica.
Best Answer
The ask is for proof of travel out of the US. It seems reasonable that an airline ticket from Costa Rica to your home country is at least as good proof that you will leave the US as a ticket from the US to your home country. To take it you would have to leave the US, and since that flight is less than 90 days from your entry date it proves you will leave before 90 days is up (sure, you could decide not to take it, but that applies to any ticket). It would be different if the flight were more than 90 days away.
However issues like this are up to the judgement of the immigration official. I would certainly be a wise precaution to have some evidence of your departure to Mexico, such as a bus ticket or a hotel booking. The more evidence you have the less likely is a refusal.
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