USPS not allowing me to ship first class

I have added first class and priority mail. To test this out, I made one of my products weigh 0.5 (which I am guessing is pounds), however, the USPS shipping module is not letting customers select first class mail when they check out; it is forcing them to select priority mail.

Did I do something wrong? What is up with this?

EDIT: I am using Zen Cart 1.3.9e.

Could you check the settings in Configuration ... Shipping/Packaging ... and see what your Tare settings are?

First Class has limits on the weight and if you have Tare settings, that will add too much ... the default is: 0:3 which adds 3lbs ... set to 0:0 to add no additional weight ...

Thank you very much for your help! It works now.

Is this supposed to be used to add the weight of the packaging to the product weight? If so, why is it set at 3 pounds?

Thank you again!

Is this supposed to be used to add the weight of the packaging to the product weight? If so, why is it set at 3 pounds?

Because we have no way of determining what the "real" tare weight should be so a random number was assigned to the defaults.

Thank you.