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Explanation: If it's a small paper or plastic bag you use bolsa. If it is a larger bag like a bag of cement, you would use saco. If it's a burlap bag or sack or other large woven item yiou would use costal or bulto.
This is the usage in Mexico.
50 lb 'costales'(if they are woven)
It might be best just to use bulto in both cases.