You can opt in on your very first purchase.
When you are checking out on our store, and you have entered your delivery / billing information and credit card information, you can save everything by selecting a checkbox and entering your mobile phone number:
The next time you go to check out on our store and enter your email address, you will receive a unique 6-digit code in a text message. Then, you will be prompted to enter the same code on the checkout page:
After entering this code, your credit card and delivery / billing information will be automatically filled out.
After you opt in to Shopify Pay, you will be able to use your saved information on our store and any Shopify store that has this feature enabled.
For more information please visit the Shopify Pay website.