Bulk mail is what the USPS refers to as commercial mail and is where mailings of large quantities qualify for discounted postage rates. Mailers will need to send at least 200 pieces (or 50 pounds or more) at time to qualify for Standard bulk mail rates or 500 pieces at a time to qualify for First Class bulk mail rates. In addition to the quantity requirements the USPS has additional guidelines which mailers must comply with to receive bulk mail discounts. Some of these requirements include presorting the mail, ensuring the address are deliverable (NCOA certification and CASS certification), filling out the appropriate mailing documentation, affixing the correct postage, and delivering the mail to the bulk mail entry unit.
Bulk mail discounts are significant, but it much easier and cost effective to have a direct mail house process your mail piece and get you the lowest postage rate possible.