I've used
K9 Capers to order food (including NatureDiet) quite a lot and always had good service, and postage is free over a certain amount.
If you want a cheaper food that's not cutting too many corners in quality, I know lots of GSD owners feed CSJ food. I used it for a while as it's what Casper was fed in the rescue, I decided to switch to raw but I probably wouldn't mind feeding CSJ again if I had to - though I've since discovered that too much lamb makes my two trump and they were both a bit stinky on the lamb CSJ so I'd probably switch to chicken! You can order direct from
CSJ direct and if you have room to store an extra bag, the postage is the same for 2 bags as it is for 1, so you save a little on postage that way.