In short, anyone can use Shop Pay. You will need to opt in at the checkout either on the Candle Shack store or on any other Shopify powered store.
When you opt in, you will need to supply the following info:
• Your email address
• Your mobile phone number (used for SMS verification)
• Your billing, shipping address
• Your card information for a valid credit / debit card, including the CVV
If you would like to opt in, click the checkbox when you are asked 'Save my information for a faster checkout' when you are checking out.