Under what circumstances will my ordinary letter/packet be charged as “Bulk Posting of Lightweight mail items (Air)”?

If customers post 1,500 items or more per day, or 5,000 items or more over a 2-week period to Croatia, Egypt, France, Germany, India, Japan, Korea, New Zealand, the Philippines, Portugal, Romania, Russia, Switzerland, Taiwan, Thailand, Türkiye, the United Arab Emirates, the United Kingdom, or Viet Nam in which the weight of each item does not exceed 150g, the posted ordinary letter/packet will be charged as “Bulk Posting of Lightweight mail items (Air)”