How would you handle this awkward situation?

I purchase fresh milk from one of my neighbours as I don’t like tetra pack milk. Getting it from them is convenient as I don’t know anyone else near by that does this.

I have an arrangement of 3 litre milk to be picked up from their house each day. But I find now that they usually give me less than 3 litres. It is usually 2.5 litres. I am on really good terms with these people and they are friends. I have mentioned to them once in passing that I don’t get all 3 litres. They said they’d look into it but then nothing happens.

How do I solve this with them as they are friends and I don’t want to stop getting this yummy milk?