Bulk mail is simply the term that describes the business practice of mailing hundreds (or thousands) of mail pieces by utilizing an approved bulk mail indicia, adhering to bulk mail requirements, and therefore, receiving a reduced cost of US  postcard stamps — which is much lower than the current cost of postcard stamps when you buy postage stamps for postcards for the holidays or vacation. Thus, there are a couple bulk mail options:
First Class Postage Postcard – gets your bulk postcards to recipients in 2-5 business days (only option for 4×6 postcard postage)
Standard Class (or 3rd Class) – gets your bulk mailers to recipients in 7-14 business days (available for 5×7 postcard postage, 5.5 x 8.5 postcard postage USPS, and 6×9 postcard postage)
Therefore, you can still receive lower bulk mail rates if you don’t want to pay for a shorter bulk mail time frame with First Class. However, note that Standard Class (or 3rd Class) postcard postage cost does not apply to the 4×6 postcard size.