People often write things the way they sound. This message still gets across, and in this case, the shopkeeper needed to put up a sign in a hurry, so it’s understandable. My problem is that major companies make mistakes like these, and yes, native speakers of English do as well.
More commonly confused words:
there – they’re – their
it’s – its
who’s – whose
Tip: don’t trust your spellchecker!