Slight confusion that the slot is 15:24 to 16:24, but I'll assume that's working on a 15:54 ETA +- 30 minutes, and 15:54 is the expected time based on which deliveries are going where today.
Oh, double kudos for both emailing and texting me with this info.
ETA: and the couriers have apparently just been, right at the start of that time slot. Truly excellent. There might be problems with the goods, of course, but the delivery is A+++.