I'm struggling with what defines a USPS First Class Mail Package. I ship small, light items and have been using some test items to find my issue. I'm using a tare of 10 (10% weight & volume (?)). I have one item that weights 0.125/0.1375 pounds and is 11.3/12.43 cu. in. which allows the First Class Package option. But another item is 0.125/0.1375 pounds and is 11.7/12.87 cu. in. and does not allow First Class Package. I don't see anything at USPS that constrains on such a small volume or weight. I'm sure I'm doing something wrong, any help would be appreciated. ZC 1.5.1. BTW: This is a much-needed add-on. Well done!