Re: [w3c/payment-request] Modifiers and [[serializedModifierData]] not actually used anywhere. (#753)

Reporting in about my action item: https://www.w3.org/Payments/WG/track/actions/103

I'll split this into two sections:

1. Amount management based on card brand / bin / networks, etc...
2. Card brand filtering (not related to modifiers but might serve as context for some)

# Amount management

We have two use cases for charging people on our platform: Shopify charges the Merchant and Merchant charges the Buyer. In both cases, we do not change the amount that is charged depending on BIN data information. Researching this a bit further (and if I remember correctly, this was mentioned on our last call for the EU), this might actually be illegal (see [here](https://www.businessinsider.com/american-express-credit-card-rewards-swipe-fees-explained-2018-6))

**Charging the merchant**

Though we don't charge the merchant different amounts depending on their card, we do manage our own BIN data table in order to do some analysis on the card being used.

**Charging the buyer**

As mentioned, we don't allow the merchant to charge differently depending on the card's BIN data. **We do however charge the merchant different fees depending on the card the buyer used.**

# Card brand filtering

For the buyer's purchase experience, we do need to filter out card brands that are either not accepted by the merchant or the payment processor. This is not done using BIN data, rather a set of regexp's akin to https://github.com/stripe/jquery.payment/blob/master/lib/jquery.payment.js#L21-L93. This is to say that `modifiers` would not be leveraged for us but `supportedNetworks` from the basic card spec would be necessary.

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