Among the area code 787, list the country of the cities with a postal point type of unique postal office.
postal point type of unique postal office refers to type = 'Unique Post Office';
SELECT DISTINCT T2.county FROM area_code AS T1 INNER JOIN country AS T2 ON T1.zip_code = T2.zip_code INNER JOIN zip_data AS T3 ON T1.zip_code = T3.zip_code WHERE T1.area_code = '787' AND T3.type = 'Unique Post Office'