UNDERTALE Folding Reusable Bag

This official UNDERTALE bag was designed by Fangamer Japan's Ikuno Ohashi and measures 10.6 inches wide by 11.8 inches tall and 7.9 inches deep.

It folds into a neat pocket-sized package when it's not in use; if you put a bunch of folded-up bags into a single unfolded one you could have nearly unlimited storage space available to you.