What does this PR do?

Sets up basic logic for parsing fee details from the passport, and displaying them to the user via the FeeBreakdown component

Design decisions

These are largely points up for discussion, I just want to explain some of my reasoning and outline potential next steps.

If we cannot parse values from the passport, do not show the breakdown to the user
Even if the showFeeBreakdown prop is set to true, the breakdown still won't show if we cannot display it to the user (e.g. missing or invalid values). If this happens it's a content issue which will be flagged via Airbrake for content editors to ultimately fix - the user journey won't be disrupted.

Hardcode application.fee data field
This is a rough MVP decision just to move forward initially, it's not a big additional task to read data.fn from the relevant Pay component here.

Reductions and exemptions are combined to a generic "reduction" value
The fee calculator doesn't associate a numerical value with exemptions and reductions, but just updates application.fee.payable and sets true/false values. There's a few options around how we could handle this -

• Update content to always set a passport value, and then remove it from application.fee.payable
• List exemptions and reduction, and sum the totals
  I'm very keen to avoid repeating any logic already handled in the fee-calculator flow - doing so puts us in an awkward code vs. low-code spot with regards to how this logic is handled.

Again, I think we take a fallback approach here. If we cannot parse a list of exemptions or reductions from the passport, we can fallback to summed generic "reductions" amount?

VAT is set via a Calculate component, and not re-calculated
I'm not sure about this one - it allows room for error if an Editor incorrectly calculates this value. A more robust approach might be to allow the user to set a application.fee.payable.includesVAT variable, and display an amount based on this.

• PR here - feat(fee-breakdown): Calculate VAT using useFeeBreakdown() #4042

Nothing is really explained to Editors
I think this will need something - I think just adding a list of variables and how this words in the Editor would achieve this.

Disabled for Invite to pay journeys
This is something I hadn't initially considered - we have no passport when returning to the service via invite to pay so the current approach does not work. We'll need to persist the fee breakdown data to the payment_requests table in a similar way to how govpay_metadata is stored. I'll open another PR shortly to provide a solution here.

Next steps...

Here's what I'd like to go for as I suspect the current implementation is too MVP -

• List reductions and exemptions, but with a single summed value each (exemptions = full amount, reductions = calculated - payable)
• Handle invite to pay journeys
• Add explanation / documentation within the Pay editor modal if showFeeBreakdown is checked