That makes sense.
I wonder how much a slot would cost compared to regular airfreight? I'm not even sure how much the regular Amazon freight service costs compared to regular truck freight.
Southwest Airlines and United both offer freight services to the public, I'm sure most others do as well. United Freight in particular has a dedicated pet transport service. Other than pets, you just walk up to the freight counter, pay the clerk and your freight arrives at the freight counter other end. Greyhound and Amtrak offer these services as well to the public. I saw a family ship a refrigerator halfway across Colombia via bus back in 2014 for $10 under a passenger bus.
If they're offering it to the public, no reason not to offer it at a corporate rate.