Making changes to the checkout experience is probably not something you'll run into very often. But there will be clients who want to customized notifications. In some situations, an app may change the checkout procedure. In all of these scenarios, it is important to test the full checkout procedure.
Fortunately, Shopify makes it easy to put through test transactions using the most popular credit cards - Visa, Mastercard, American Express, Discover, Diners Club, and JCB.
Usefully, you can also simulate failed transactions to test what happens when a card is declined, disputed, expired, or invalid for a variety of other reasons.
There are two guides on Shopify's site related to this. One for "Placing a test order" and another for "Testing Shopify Payments." Both are immensely useful.