To allow Shopify to send emails on your behalf you will have to add it to your SPF record. The SPF record mechanism used for Shopify is shown below.
To setup DKIM for Shopify, you can authenticate your domain by clicking Authenticate on the Sender email page of your Shopify admin console. A new window will open and provide you with instructions on how to create four new records with your domain provider.
Create a free OnDMARC account to test your configuration.