If I remember correctly If you buy it at the so called duty-free shop in the SJO airport they will get it put onto the plane, which I assume means no payment from the passenger for an extra bag.
While down there my last time 2 yrs ago I was toying with buying a couple of cases for a friend, didnt in the end as I wasnt sure which type he would have wanted. But the guy said they take care of getting it onto the plane. So amybe you pay alittle more but at least you wont have to pay for any extra baggage. id do it on the way back rather than on the way to where ever Im heading.