transport: note SIXT counter payment

bookings/transport/car-rental.md

@@ -14,7 +14,7 @@
 | **Booked duration** | 15 days |
 | **Route** | SFO → Yosemite → Sequoia / Kings Canyon → Los Angeles → Las Vegas → SFO |
 | **Price** | US$897.93 including taxes and fees |
-| **Payment status** | Not stated in confirmation |
+| **Payment** | US$897.93 due at the counter at pickup |
 | **Refundable hold** | US$200 at pickup — not included as an expense |
 | **Mileage** | Unlimited |
 | **Roadside assistance** | Included |