According to the United States Postal Service (USPS), bulk mail is “quantities of mail prepared for mailing at reduced postage rates.” To obtain bulk mail discount rates, mailers must register for a bulk mail account with the USPS and adhere to bulk mail guidelines (including use of unique indicia, labels, imprints, etc.), though no stamp is required.
Bulk mail marketing is the most common way to take advantage of the discounted postage, though in some cases it can be advantageous for personal use (i.e. wedding invitations). It is possible to get bulk mail rates on postcards and letters, though postcards are the most common and cost-efficient option.