How to get back to the Undyne area after going to Snowdin

If you lose to Papyrus when challenging him on a peaceful run, he will put you inside the shed. There's not much of interest in there.

There's nothing stopping you from leaving the final area before the Asgore fight.

In fact, before the first attempt at the fight, Asgore even gives you an opportunity to go back for anything else you needed to do. I'm not sure how you're getting lost going back, since it's a completely linear path back to the Core other than the elevator shortcut.

Either take the first elevator you see and then continue south until you reach the elevator back to the Core, or ignore the elevators until there's nowhere left to walk. This will take you back to the end of the Core. The elevator just outside the Mettaton battle area will lead you back to the first room of the Core. Simply continue south from there to get back to MTT Resort, where you can find a shop, or continue out elsewhere for other shopping options or sidequest activities.