Auntie doesn't have particularly good news for you -- because iTunes for OS X fails when it comes to resuming shuffled playlist playbacks. Here are a couple of scenarios for you:
Scenario 1: You start playlist playback, hop into the store and click the Stop button. Playback ends, and iTunes has lost your place. You can't resume playback.
Scenario 2: You stop playback while in your playlist and hop into the store. Again, iTunes has lost your place. You can't resume playback even if you pop back to your original playlist.
If you own an Apple-branded keyboard with the built-in play-pause key, your options improve. While browsing the store, you can pause and resume your music without iTunes losing your place -- although you might be amused at what the iTunes interface does as you're clicking that play-pause button (watch the top-left of the iTunes window as you click).
The bad news is this: if you use that exact same play-pause key to pause playback and take a call before hopping into the store, you're once again out of luck. Paused music is automatically cleared as soon as you enter the store.
iTunes doesn't really hate you -- it's just built that way.
You might imagine that there's some kind of preferences fix or a way to use AppleScript to work around this problem, but there isn't. While you can use AppleScript or osascript to 'tell application "iTunes" to playpause' (handy if you don't own an Apple keyboard), the application will not 'resume' when prompted after a hard stop. Oh, how Auntie tried.
This morning Auntie dumped app symbols, searched through its strings, read its AppleScript scripting dictionary and more. In the end, she tossed up her hands in defeat. As far as she can tell, there is no reliable way to ensure that you can resume a shuffled playlist from the point where you paused.
Using the keyboard play-pause button is really your best bet, but there's still a good chance that iTunes will mess you up if you need to hit the store while paused.
Sorry about that.