As Shopify partners, all payments for our themes are handled by Shopify. When a theme is purchased from the Shopify Theme Store, it is considered to be final sale. This is due to various reasons including copyright licensing as well as themes being considered to be non-tangible, irrevocable digital goods.
While we can completely understand how that may not be ideal, you are given the opportunity to try out a theme before committing to its purchase. To learn more about these restrictions, you can check out Shopify's Terms of Service where it outlines their refund policy.