Say it in the agreement, before the cake is baked.
In my custom cake agreement, once the cake is delivered or picked up, the business is released from liability for its condition. Transport and climate-controlled storage are the client’s responsibility.
For wedding cakes the clause is more specific. The client has to have a level, stable, climate-controlled place ready when you arrive. When setup is done, you take “Proof of Condition” photos, and from that point the client is responsible. The agreement also says the bakery is not liable for damage from heat, humidity or handling by venue staff or guests.
The photos matter. They show what you handed over, in case anyone later says it arrived damaged.
This is how my own agreements work, not legal advice. Check what applies where you bake.