Among the types of postal points in Saint Croix, what percentage of postal points is the post office?
DIVIDE(COUNT(type = 'Post Office' ), COUNT(type)) as percentage where county = 'SAINT CROIX';
SELECT CAST(COUNT(CASE WHEN T2.type = 'Post Office' THEN T1.zip_code ELSE NULL END) AS REAL) * 100 / COUNT(T1.zip_code) FROM country AS T1 INNER JOIN zip_data AS T2 ON T1.zip_code = T2.zip_code WHERE T1.county = 'SAINT CROIX'