Bundled with a unitypackage, this script will search for PACKNAME and load an editor window when imported.

Package Import Loader

class MyPackInstaller : EditorWindow 
{ 
	public static void Init() 
	{ 
		EditorWindow.GetWindow<MyPackInstaller>().Show(); 
	} 

	void OnGUI() 
	{ 
		// code to load pack with options here 
	} 
} 

class MyPackagePostProcessor : AssetPostprocessor 
{ 
	const string PACKNAME = "MyPackage";

	static void OnPostprocessAllAssets ( 
	string[] importedAssets, 
	string[] deletedAssets, 
	string[] movedAssets, 
	string[] movedFromAssetPaths) 
	{ 
		foreach(var str in importedAssets) 
		{ 
			// Detect a new unity package 
			if(str.Contains(".unitypackage") && str.Contains(PACKNAME)) 
			{ 
				MyPackInstaller.Init();
				break; 
			} 
		} 
	} 
}