I have been stuck on this level for a long time.  I actually quit playing the game, because the closest I have come is 3900.  I chain as much as I can, I give toys and collars with cats/dogs and fish.  when they are open to anything, I choose the rainbow bird or a dog or cat, and always do cages for all animals that can use them.  Fish I put in aquariums with rocks.    I can't get past it!! 
 
So, for those that have - any suggestions??

I was able to pass this level and get expert by only giving customers the rocks, cages, or vet if they asked for it.  I basically served customers with the animals they asked for as soon as possible.  If they asked for any animal then I would give them the most expensive one which is the rainbow bird.  If they asked for any fish, I would also give them the most expensive, the rainbow fish. 
 
Also, if I had time I would add the dog/cat collar or fish accessory to an order to make more money.