Adding a markup to your carrier rates is very simple in Shopify.
Follow these steps to modify your rates:
1. Navigate to your Shopify admin and then to the Settings panel.
2. Click through to Shipping from the tile grid.
3. Select the shipping zone which you wish to markup and click the edit link.
4. Scroll down to the "Calculated Rates" group and click edit.
5. Add your positive/negative rate adjustments in the desired fields.
You have successfully modified your rates.