CheckoutFundingSource

Which funding source the buyer picked in the Sessions-owned toggle above the Stripe Payment Element.

This is a request, not an instruction. The server re-derives whether bank debit is genuinely on offer for the specific charge — operator toggle, rail, connected-account capability, minimum amount, lead time — and silently falls back to CARD when any gate says no. Silently, because the buyer's alternative is a card they can use right now: an error would strand a checkout over a condition they can't see or fix.