Re: [w3c/browser-payment-api] Currency Types and Rendering (#185)

An interesting thought experiment is that if we say amounts are defined per payment method then this becomes much easier (potentially) - see #4 

Unusual currencies will likely be specific to a payment method so the payment method spec would define what currency identifier to use and we also have far less chance of conflicts (so no need for URLs).

The challenge remains, how to display amounts in that currency. For this I think we need input from the browsers how they would display amounts if there were different amounts per payment method because that could impact this question.

If a payment request has the following data:

| Payment Method | Currency | Amount  |
|:--------------:|----------|---------|
|   Debit Card   | USD      | 1       |
|   Credit Card  | USD      | 1.10    |
|     Bitcoin    | BTC      | 0.00001 |
| Store Loyalty  | Points   | 1000    |

And the user has 3 payment apps installed

| Payment App | Supported Methods |
|:--------------:|----------|
|   App 1   | Debit Card, Credit Card|
|   App 2  | Bitcoin    |
|   App 3 | Bitcoin, Debit Card |
| App 4 | Store Loyalty, Credit Card |

1. How would this be shown in the items list (if at all)?
2. What would the app selection look like?

Perhaps the answer is that the request MUST specify a reference amount in a currency that uses a 3-letter code (ISO or not, it doesn't matter). This is used for display in the items list and where there is a different currency/amount per app this is displayed in the app selector BUT if there is more than one possible amount for an app then no amount is shown.

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