Traveling from the US and took the ferry from UK to Saint Malo, so booked the car online a few days before. Unlike US rental cars, booking won't be completed immediately. You need to wait at least a day to confirm. My initial booking request was rejected with unknown reason, so I changed the requested car and rebook, then it worked.
Counter location is in the ferry terminal, so very convenient. I booked at 9AM because Google said it opens 8:30AM, but it didn't open until 9AM (or possibly they open a bit earlier in high-season).
The guy showed up at the counter, Nalo (? I couldn't hear his name well, but young nice guy wearing glasses) was really nice and very polite. Process took a bit probably because I have non-local driver's license, but nothing troublesome. His English was also good so no communication issue.
The car was parked in the parking lot next to the ferry terminal. Car was smelly but almost new car, so overall I was very happy with Sixt rental car in general and this specific location.