fix(billing): fall back to legacy default source when no compatible payment method

enterprise/app/services/enterprise/billing/default_payment_method_reconciler.rb

@@ -1,28 +1,28 @@
 # Ensures the customer's default Stripe payment method can actually bill the given currency. PIX/boleto
 # are BRL-only, so when an account moves to a currency they can't pay we drop them as the default and
 # pick a compatible method (a card) if one is attached. Incompatible methods stay attached for later.
-# Keyed off invoice_settings.default_payment_method, since that's what Stripe Billing charges.
+# Keyed off invoice_settings.default_payment_method (what Stripe Billing charges), inspecting it before
+# falling back to a legacy default_source card so an incompatible invoice default can't be masked.
 class Enterprise::Billing::DefaultPaymentMethodReconciler
   pattr_initialize [:account!, :currency!]
 
   # Returns the id of a currency-compatible default payment method, or nil if none is available.
   def reconcile
-    current_default = current_default_payment_method
+    customer = Stripe::Customer.retrieve(stripe_customer_id)
+    current_default = customer.invoice_settings.default_payment_method
     return current_default if compatible?(payment_methods.find { |method| method.id == current_default })
 
     compatible = payment_methods.find { |method| compatible?(method) }
     return make_default(compatible.id) if compatible
 
+    # No compatible attached PaymentMethod. Drop an incompatible invoice default so Stripe won't charge
+    # it, then fall back to a legacy default_source card (currency-agnostic) the invoice can still use.
     unset_default if current_default.present?
-    nil
+    customer.default_source.presence
   end
 
   private
 
-  def current_default_payment_method
-    Stripe::Customer.retrieve(stripe_customer_id).invoice_settings.default_payment_method
-  end
-
   def payment_methods
     @payment_methods ||= Stripe::PaymentMethod.list(customer: stripe_customer_id, limit: 100).data
   end

spec/enterprise/services/enterprise/billing/topup_checkout_service_spec.rb

@@ -54,6 +54,15 @@ describe Enterprise::Billing::TopupCheckoutService do
       expect(account.reload.limits['captain_responses']).to eq(1500)
     end
 
+    it 'charges via a legacy default source when there is no invoice default or payment method' do
+      allow(Stripe::Customer).to receive(:retrieve).and_return(
+        Struct.new(:invoice_settings, :default_source).new(Struct.new(:default_payment_method).new(nil), 'card_legacy')
+      )
+      allow(Stripe::PaymentMethod).to receive(:list).and_return(Struct.new(:data).new([]))
+
+      expect(service.create_checkout_session(credits: 1000)[:credits]).to eq(1000)
+    end
+
     it 'raises error for invalid credits' do
       expect { service.create_checkout_session(credits: 500) }.to raise_error do |error|
         expect(error.class.name).to eq('Enterprise::Billing::TopupCheckoutService::Error')