Your shopping cart should provide you the shipping rate API.
Acutrack uses USPS or FedEx API to estimate the shipping cost. You should be able to speak to your eCommerce company and ask for access to the same API so you correctly calculate the shipping cost and bill your end-users.
You may need to set up an account with USPS or FedEx to have access to their pricing. You may then configure your shipping cost strategy.
Alternatively, if you want to access USPS API and integrate into your eCommerce platform, refer to this link:https://www.usps.com/business/web-tools-apis/distributor-guides.htm