To use a manual shipping method as a shipping rule, you need to first create a custom shipping method.
Create a Custom Shipping Method
Go to Connection > Shipping Carriers
Click Add Carrier and select Custom shipping method
Choose the name for the custom shipping method
Create a Shipping Rule
Go to Settings > Shipping Rules > New Shipping Rule
Define your requirements (e.g., weight, destination, SKU, etc.)
Under Shipping Method, select the custom method you created (Mailstamp)
Once saved, the rule will automatically apply the Custom shipping method to orders that match the conditions you specified.