Calls To Action (CTAs) have already been covered as an important part of landing pages and product pages. And guess what? They’re also an important part of your checkout flow. Especially as they’re going to appear a few times on a multi-step checkout.
As well as following the other rules, such as being in a clear contrast colour to the rest of the page, if you’ve got a CTA on each step they should look and read the same. That means not changing the wording for each step of a checkout flow, as it requires extra thought from the user to interpret what it does.
Think of your CTAs as training the user into what to expect: if the first two steps use the word 'Next' or 'Continue' you should keep that the same on the third, as you've built the expectation that this will take them forward in the process. Changing it to say 'Pay' or adding extra copy underneath to reassure actually often does the opposite and causes some users to question rather than act instinctively. I've pushed up the conversion of a checkout step by 25% by changing a differing CTA to match with the others in the flow.
The exception is if you're sending users to another site to pay. This is something you should make clear so they aren’t shocked when it happens.