速達 (express delivery) + 便 (a mail or transport service). 速達 alone is used just as often; 速達便 stresses that it is a particular service option.
Common collocations
- 速達便で送る: to send by express mail
- 速達便にする: to make it express mail
- 速達便料金: the express mail fee
Similar words
- 書留: registered mail — about tracking and compensation, not speed; often combined with express
- 宅配便: parcel delivery service — run by private couriers, for packages rather than letters