우편 = postal delivery service.

준등기= priority mail/delivery.  (준 means semi- or pseudo-, so it refers to a service class just below registered mail).

등기 = registered mail/delivery. (the most secure service with full tracking)

(준등기 and 등기 are traditional terms for service types of postal mail - their full names are 등기 우편 and 준등기 우편).

택배 = personalized delivery service. (This refers to recently popularized home delivery (especially for online orders) which offer a faster and more flexible service).