The first thing you need do is register for a Shopify Partners account. After you have accomplished that, you will have the ability to establish what are known as "Development stores." These stores are EXACTLY the same as standard stores, except that they are free permanently. However, once you've reached 50 sales on it, you will need to begin making payments. However, by that time, you will have made 10 times that monthly plan payment regardless, so it doesn't really matter which one you choose.
 
This is a famous trick that I picked up years ago and have been employing ever since. Once you have reached fifty sales, you can even take it a step further and just create a second store under the same domain name. However, it's possible that you don't have enough time to add all of the products and recreate the design, etc. It is up to you to decide.