You will find information on how to setup your shipping rates with Shopify below; however, please take a look at the instructions provided by Shopify first.
You may have noticed that charging an exact flat rate on Shopify is quite tricky, if not impossible. We provide you with an alternative solution, which you will win in some cases and lose in others. In the end, it should even out.
This blog post contains an example of how to calculate your shipping costs based on the arbitrary weight of your items.