15-15 Prox 1.2 payment terms means that any product shipped/received from the 15th of one month to the 15th of the next month will be paid on the 2nd month of the 2nd day. For an example, anything shipped between July 15 ~ Aug 15 will be paid on October 2. It would be similar to Net 45 ~ 75 days. Items shipped on Aug. 15 would get paid in 45 days and items shippped on July 15 will be paid in 75 days. It behooves you to ship as much as close to the 14th as possible.