What is an ordinary letter? Is this an ordinary letter?

Surface mail is a kind of letter sent by ordinary mail, not a specific type of letter. Compared with express mail, registered mail and other mails with urgent or special services, it is an ordinary letter. When sending ordinary letters, you need to use standard envelopes. The advantage of ordinary mail is that the postage is relatively low, but the mail does not provide a receipt, does not accept inquiries and is not responsible for compensation.