In case both items are in stock?
Given that both items are currently in stock, it's a no-brainer: it simply makes sense to receive both items in a single package. This is the easiest option for the consumer and the most cost-effective option for you as a retailer. Moreover, it is also the most sustainable solution.
In case only the dog food is in stock?
The dog is hungry and in need of food. In this situation, the consumer would most likely prefer to receive the food as soon as possible rather than wait for the toy to be restocked. As a consumer, it is probable that receiving two packages instead of one is less of a hassle than having to purchase another bag of dog food elsewhere.
In case only the dog toy is in stock?
If the retailer is only able to ship the dog toy, the consumer wants to be informed as soon as possible. They want to have the option to consider their choices. Is there an urgent need for the toy, or would it be more convenient to receive a single package? At the very least, the consumer expects a sincere apology from the retailer.