When you are scheduling your mailing you are presented with a choice of several classes of mail. Here is a brief look at the pros and cons of each.
Choose First-Class Mail for time-sensitive mail that you need to have delivered between 2 and 4 days after it is presented to the Post Office. (USPS delivery times are not guaranteed but First-Class Mail typically is delivered in 1 to 4 days of acceptance at a Post Office.)
First-Class Mail is required for bills, statements of account or invoices, personal correspondence, personalized business correspondence, and all matter sealed or otherwise closed against inspection.