Selfnamed selects the shipping carriers and sets the shipping rates for all orders.
It is not possible to choose a different carrier or a lower rate on your end.
That said, if you would like to offer cheaper shipping prices to your customers in your store, you are welcome to set up custom shipping profiles and charge your customers a lower rate. In that case, the difference between your custom rate and Selfnamed's shipping cost would need to be covered from your product markup.
If you have any further questions, feel free to reach out to our support team.