[galvin]

There are reduced rates of VAT for certain categories of products and services so payment processors would need to know what the charge they are processing is for.

It's not uncommon for invoices to include multiple VAT rates in certain industries so that would add another level of complexity.

A grocery store will charge different rates for essentials and non-essentials, a builder will charge different rates for materials and services, and there are probably other examples that apply to online sales too.