Flats mailings are very different from letters and cards. The rates are based on the packages, not on the trays or sacks. If you had at least 10 pieces going to ZIP code 94301, for example, they would be rubber-banded together into a 5-digit package and charged the 3/5 rate. If you didn’t have enough 94301 pieces to make up a full 5-digit tray or sack, that package would be put in a 3-digit, ADC, or MADC tray or sack--but that package would still be charged the 3/5 rate.