What is the informal term often used to refer to the US Postal Service?

The term "snail mail" is informally used to refer to the US Postal Service due to the slower speed of traditional mail delivery compared to electronic communication methods such as email. The imagery associated with the word "snail" highlights the contrast in speed—while emails are delivered almost instantaneously, physical mail can take days or weeks to arrive. This term serves to underscore the inefficiency of mail in comparison to modern digital communication, often being used humorously or colloquially to differentiate between the two types of correspondence.

The other options do not capture this contrast in speed or have a direct association with the Postal Service. For instance, "Fast Mail" and "Express Mail" might imply delivery services that prioritize speed but do not reflect the general connotation of traditional postal services. "Online Mail" suggests digital communication and does not relate to postal services at all. Therefore, "snail mail" is the most fitting informal term for the US Postal Service.