Found the issue - the “Update Payment Information API” cannot be used to update the transaction amount or other transaction details. It is used only to update vco with the status of the checkout transaction.
There is a different API “updateIntent” to set the transaction total. This is usually invoked only once via the UI screens(behind the checkout widget). Once this is done the transaction total cannot be changed anymore.
The Checkout product as it currently stands does not have the capability to perform multiple “updateIntents” per checkout session.
Does this help?
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When working in a sandbox account with our payment gateway, it seems to process the charge just fine (there is no dashboard by which I can actually verfiy this though).
Could the gateway doing something behind the scenes that actually updates the transaction total? Would visa checkout return them an error response if the charge was initialized at $100, but charged at $105 because of a $5 delivery fee?