Following on from #684, there are a bunch of huge issues in the spec relating to modifiers:

• [[serializedModifierData]] is not actually used anywhere - it's undefined how that data actually gets applied/displayed.
• [[serializedModifierData]] gathers all the data from EVERY modifier (without matching on PMI!), then passes ALL data to the chosen payment handler - in the spec, there is no way to distinguish between data for ApplePay and data for Basic-Card.
• The modifiers in serializedModifierData are never converted, only serialized, meaning that if they are converted by the payment handler, they can throw: This is undefined behavior.
• The actual serialization also seems pointless. If that's for IPC, then that should be an implementation detail. It doesn't seem to serve any purpose in the spec (because [[serializedModifierData]] is never actually used for anything).

I'm again strongly inclined towards dropping modifiers in favor of something else. They are super broken, underspecified, and unnecessarily complicated.