I noticed PayPal doesn't take the order weight total during payment. For an order placed with PayPal, it'll show a total weight of 8 oz even though the actual order weight could be 20 lbs. This causes me to lose money on shipping.
I do use the Total Weight of Order add-on from WebShopAddons. Could this be the problem?
Shipping charge calculation has nothing to do with PayPal. All calculations are done on your end then sent to PayPal. I'm not familiar with the addon in question so you will need to ask the developer if it interferes with shipping calculations.
Other than that, you need to check your configuration.
Well the developer says the same thing - that it can't be their addon, so it must be CS-Cart. There is nothing in my PayPal configuration to change that could possibly cause this.