Take milk from cows to the dairy production chains building.
You might want to pay the cheese factory, chocolatier or the bakery a visit. If you provide them with all the necessary ingredients for products like chocolate, including the milk, you can make even more money compared to just selling the milk.
Use Lizard MKS8 or MKS32 to transport milk from dairy cows to production chains dairy building, selling point or where ever you need to.