To set up specifically charged or free shipping, it is a two-part process which involves both your Shopify admin and ReCharge Shipping Settings. You will configure shipping rates by weight, then change the product weight to correspond.
Warning: In your Shopify Shipping settings, if you're wanting to use weight-based Shipping, packages can be a source of trouble, since they throw off the total weight of the product.
Set Up Weight-Based Shipping
From the Shopify Dashboard go to Settings > Shipping and click Edit next to the shipping zone to include the new rate. If you don't have one already, you will need to create one in step 2.
In the section titled Weight based rates, click on Add Rate.
Specify a Name, Minimum & Maximum order weight, select free or specify a rate then click Done.
Warning: If you make the weight range very narrow, please take into account that minor rounding is taking place. Shopify views rulesets in lbs but the actual weight in the cart token is grams so Shopify converts it, this results in minor rounding errors that cause narrow weight ranges to sometimes be affected.
Don't forget to Save the rate once you have added all of the weight-based shipping rates. You can add multiple rates in the same zone if you need to specify free shipping and flat rates.
Add the new corresponding rates to the products in Shopify by opening the product and scrolling down to Shipping and adding in the new weight and Save.
After you do this, you will then need to go into your ReCharge shipping settings and make sure that the Charge Shipping radio is selected. This ensures that your rates are pulled from Shopify.