As a donor, I want to first select my currency, then only be presented with payment gateways that support that currency. This is a flow that should not happen:
  1. Form supports INR and USD, defaults to USD and PayPal Standard on page load.
  2. When a donor selects INR from the dropdown, they are immediately presented with an error message "INR is not supported in PayPal Standard" or similar.
This is backward. Currency should drive available gateways. If a donor wants to use INR, only the gateways that support INR should be presented as options.