Write the checkout step in pseudocode. No framework.
Answer it out loud before you open anything. The value of the flags below is in comparing them to what you actually said — including whether you asked who uses it before naming what you would build it with.
The situation behind the question
The interviewer hands you a marker: "Write the server side of 'Pay' for the store in pseudocode — inputs, outputs, state, branches and failures. Not a language, not a framework. Then tell me what you learned by writing it."
What it is really testing
Whether the candidate uses pseudocode as a thinking tool — to find the branches and the state they had not considered — rather than as a lower-resolution version of the code they were going to write anyway.