First Class mail is delivered faster by the USPS than Bulk mail.
First Class mail is generally delivered in 1-3 days locally and 3-5 days nationally by the USPS. First Class mail is also more expensive than Bulk Rate mail. Rates are based on size and weight. Certain types of mail must be sent via First Class mail, such as invoices, statements, and other personalized mail. First Class mail is also forwarded automatically or returned to the sender if it is undeliverable. First Class mail is generally perceived to be of higher importance by the recipient, so it generally gets looked at more closely by the recipient.
Bulk mail is generally delivered by the USPS in 2-5 days locally and 1–3 weeks nationally, but in exchange for the slower delivery, Bulk Mail is less expensive than First Class mail. Bulk mail is not automatically forwarded or returned to the sender if it is undeliverable (though you can pay extra to USPS to receive these services).
Bulk mail can’t be used to mail invoices, statements, or some other kinds of personal mail. Bulk mail rates are also based on the size and weight of the mail piece. There is a minimum quantity of 200 pieces of like size and weight in order to mail at bulk rates.