The ms company that shops post office sends you a small package (containing plain white paper) and you mail this package from the post office. You do not mail your own stuff for these shops with this company. There are other companies that involve mailing bags of rice to an address that's either a soup kitchen or food bank, depending on which address they give you.
Her Serene Majesty, Cettie - Goat Queen of Zoltar, Sublime Empress of Her Caprine Domain