They probably sold the other 3 at once or sold 2 and one died or something, I don't know. either way they shouldn't be selling them singly. You don't have to buy 2 cages, it's just what I do for mine How much is he? He is a male right? and yes they do get very depressed without another one and can die.

Which cage was it?

Guinea pig nuggets can also work, I just use chinchilla pellets as from other shops I can get bigger bags for cheaper and the degus still love it, though P@H do sell degu food now but it's a bit expensive!

Peace
Akai-Chan