Shopify script to filter country-specific payment methods

filter_gateways.rb

def allow_in(gateway, eligible_country_codes)
  if Input.cart.shipping_address and eligible_country_codes.include?(Input.cart.shipping_address.country_code)
    Output.payment_gateways = Input.payment_gateways
  else
    Output.payment_gateways = Input.payment_gateways.delete_if do |payment_gateway|
      payment_gateway.name == gateway
    end
  end
end

allow_in('American Express', ['US', 'CA'])