The other day, we were reminded of the 1922 novelty song by Frank Silver and Irving Cohn from the Broadway revue Make It Snappy, “Yes! We Have No Bananas.”
In this case, “Yes! We Have No Grammar” might be more appropriate for this Kroger gas station. Here’s the display we had to see while filling up the tank. Ouch!
Grammar Police a.k.a. GrammarCops 
Alas, the misplaced apostrophe is becoming almost a lost battle– Today I’ve seen, by someone who’s just thinking “possessive”: Your’s, Our’s & Her’s! What’s next, His’s?
I believe the correct possessive is Hi’s.
Maybe Hi’s for His or His’s, but then what do you do for the incorrect plural of an informal greeting … formal: Hello’s and informal: Hi’s … all tongue in cheek, of course.
I think you have this one all wrong. Maybe they are actually selling single bananas for $.89. All they’re doing is letting us know that banana is available for $.89! Or it could be that they have an employee who will clean your windows for $.89 and his nickname happens to be “Banana,” and they want the customers to know that Banana is available for $.89.