I use USPS priority mail but am charged for a box that is not on drop down menu

Question

I use USPS priority mail boxes but keep getting charged for overages because the label doesn't match the service. The box I use isn't in the drop down menu, for instance, I use a box that is 12 by12 by 6 and I look through the drop down menu but the only thing that fits is USPS Priority mail. What am I doing wrong?

disneyshopper · Answer

When you go to select your shipping service, select custom size and not the USPS sizing, and calculated shipping.