I don’t use RCP, I use Ecwid but I found this, From the RCP manual:
Set Up Shipping Fees
If at least a product version configured a Physical, shipping is added to cart, during checkout RapidCart Pro shows a shipping method picker.
Like costs and discounts, shipping methods are calculated as a Fixed or Percentage amount using a rule-based logic. Setting the shipping rule Type you can choose if shipping fees are calculated using the number of Items in cart, order Weight or Total amount. You can define a set of Steps and associated amounts. If you prefer offering free shipping over a total amount, check Free shipping over total amount of and enter desired amount in the text field.
RapidCart Pro does’t integrate with online shipping calculator by UPS, FedEx or other courier delivery services company. Shipping fee calculation is based only on rules set in plugin.
Countries control in Shipping panel can be used to limit the availability of a given shipping method to some countries.
If you don not enter any values into Countries control, shipping methods will be available for all countries.