"Shochu Omimai Moshi Agemasu" and "Zansho Omimai Moshi Agemasu" are first phrases to write in postcards. Japanese write postcards to ask friends how he or she has been getting along in summer. "Shochu" 暑中 means the hot period from the end of rainy season to Liqiu (August 7 in this year). and "Zansho" 残暑 means the heat after Liqiu. So we write "Shochu Mimai" before Liqiu and "Zansho Mimai" (We call each postcards as "Shochu Mimai " and "Zansho Mimai".)
I have received a Shoshu Mimai from my friend this year. I was very glad. Receiving a letter or postcard is glad and special for us. (Do you think so ?) As I mentioned before, we seldom write a letter now. However, I think that this is good custom.
This is image of Shoshu Mimai