When purchasing products online, it is important to make sure payment process works and will not fustrate the user. Therefore, implementing the correct code to make it work and making sure the instructions are correct when testing with dummy payments.
Check out the Craft 2 Commerce 1 documentation and Craft 3 Commerce 2 documentation of how it works.
Check out the Shopify documentation and setting up a Shopify store of how it works.
Check out the cheat sheet for Shopify liquid
15th Jan 2019
Craft Team