woocommerce check out errors with usps

I have a new yet-to-be launched site built on woocommerce flexishop2. I am trying to keep it simple and just have usps shipping but it is too complicated to know how to break up the products so that they will work with the shipping. And the check out doesn't work either. I need someone with experience in usps shipping extension, how it is set up successfully with creating the product dimensions so that shipments cost the right amount. I also need someone who has experience making it all work so that checkout functions. Right now I am getting errors. I'm sure this is very simple for someone who knows what they are doing.