Penny payments will never* be viable.

* Never say never. Basically I don't think it could possibly be viable because it would require the total cost of transaction to be less than pennies. That includes the shopkeepers time to manage the automated store, the electricity used in the transaction, and a bunch of other non zero components.

Hell even transactions with real pennies today are almost a net loss. We eradicated 5c coins in NZ because they cost something like 6c to make.

And to be clear I understand you mean micropayments in general and maybe it is something possible im just not yet convinced the electricity costs will allow it unless it's subsidized in an unsustainable way.