Efficiency and speed are everything in mail service.
Mail has convenience because it is sent for you in the mail.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. The reading of 便 (びん) in this vocabulary is an exceptional reading, so here's a mnemonic to help you out. You get a lot of junk mail so you throw it in the waste bin (びん). The waste bin is where junk mail goes.