what is first class mail postage refers to the cost of sending letters and lightweight packages through the USPS (United States Postal Service) using its First-Class Mail service. First-Class Mail is the most affordable option for sending small parcels and letters, with delivery times typically ranging from one to five business days, depending on the destination. First-Class Mail postage rates are based on the weight and size of the item being sent. For example, sending a letter in a standard envelope starts at around $0.66, while a small package can cost more, depending on its weight and size.