No, you do not look pregnant, but neither do you look 58 years.;)
I see the good side is the drug's Fountain of Youth effect.;)
The bad side is the gender change (but if it turns out as it did for you, maybe that should be added to the good side, too.:D
Seriously, you have been given a lot of sensible propositions and also some accurate legal advice.
Hypothetically, were I a lawyer, I would be considering Driver's initial propisition about offer, acceptance and consideration and then thinking that the first two transactions were completed.
The idea of "mistake" means that you cannot wrongly profit by someone's error when you know they have acted on a wrong basis, even moreso if you have encouraged their erroneous opinion. Akin to that is the story of a Queensland lawyer being fined $20,000 by not revealing that his client had a reduced life expectancy because of cancer that was unrelated to his claim. The "silence" should have been bad enough for the lawyer to be wrong, but he went further and was stating that his client would have been fine, but for what had given rise to his claim.
Assuming you had no knowledge of what the correctprice shouls have been and/or assumed it was at the usual sort of PBS price, you had no problem and the chemist would not entitled to attempt to charge after the event, i.e. after the contract had been completed and you had consumed the supply.
There should be a professional body that "polices" the conduct of pharmacists in your state. The Guild is more a voluntary but (I think) national association as I recall their role. If you need to write to a body, it should be to your state's pharmacy policing body because of its role in maintaining professional standards.
BTW, there are diffferent regimes by which non-PBS drugs are sometimes available. If we are talking about a small community, your GP should separately speak to the pharmacist to assist; obviously, big city folk are not as kindly disposed, usually.
Lastly, as an aside on the consumer purchases issue, the goods displayed in the shop are not necessarily said to be offered to the customer. The "offer" is more usually when the customer seeks to purchase them. Of course, the Trade Practices Act and the ACCC have put refinements on these situations. If a store advertises specific goods for sale at a price (without saying that perhaps only a few are available), when they do not have enough to satisfy consumer demand, they can be potentially fined for what is called "bait advertising".
Hope some of this is of benefit.