your .is domain easily connects to Shopify via ISNIC's forwarding service. Notice that if the domain is hosted on other name servers then the following instructions do not apply.
First step is to set up the domain in Shopify, see Shopify's instructions. There under online store you can find domains and there is connect existing domain.
In order for the .is domain to connect to your Shopify website the domain has to be configured on isnic.is.
In the top text field where it says Complete URL or IP address you insert the IP number „23.227.38.65“ and then „shops.myshopify.com“ where it says CNAME record, according to Shopify's instructions. Then finish by clicking the submit button.
The domain should be correctly set up now and you will only need to check if Shopify will confirm the afformentioned changes. Go to the verification page and run the test. Notice that you might need to wait until Shopify can see the updated domain information.