Customers being incorrectly charged for shipping

My site is for my boss’s coffee roasting business. When customers buy a bag of coffee, it is supposed to generate shipping costs for them. Starting last week, some people were being overchanged about 33% more than what they should be charged for shipping and some people were charged about 50% less than what they should have been. We use Woocommerce for our store and ship with USPS. I have tried creating shipping zones, but there is no option to use USPS so I stopped with that.
My current theory is that the weights are different online and in real life, but they seem to look right in each product. Is there a separate place I need to add weights in the USPS settings?
Has anyone else had this problem? Thanks in advance for any help.
Here is the site: jackiesjava.com