Unity Purchasing already has same MiniJSON declaration.

[Slava Afanasyev]

Lib (contains MiniJSON.cs, for parsing. NB: We've had one report of a namespace clash with MiniJSON...presumably because more than one library is using it. Please let us know if you encounter the same issue.)

[Marc Tanenbaum]

Sadly this is a fact of the current inability of Unity to do proper package management. I believe IAP have fixed this in 1.13.0...which is not to say that we shouldn't do something similar on ours.

[Slava Afanasyev]

yes, new version of iap package now is using UnityEngine.Purchasing.MiniJson namespace, issue can be closed