Hello,
Your issue can be coming from the fact that you only set a weight for one of your products and no dimensions for any of your products, you'll NEED to set dimensions to your products else you'll have to customize your USPS shipping plugin to avoid setting your product dimensions.
Also, you are right, there is a small issue on the USPS shipping plugin which prevent us to send multiple packages to USPS, I'll fix it on my end and if you want a version which doesn't need any product dimensions just tell me and I'll send it to you as fast as I can after I fixed that issue.