"Prices displayed by a business must be clear, accurate and not misleading to consumers. You should always display the total price of a product or service. When you present prices to your customers, you must state the total price of the good or service as a single figure, which is the minimum total cost that is able to be calculated."
It's illegal not to comply.
Interesting. Suppose you go into a fast food place and say "I'll have a burger, medium onion rings, and a small soft drink". Assume onion rings come in medium and large, soft drinks come in small, medium, and large, and fries come in small, medium, and large. Assume onion rings and fries of the same size cost the same, and one can be substituted for the other in a combo with no price change.
The clerk could ring this up as those three separate items as ordered, giving price P1.
A second option is to ring it up as a medium combo (Burger, medium fries, medium drink), substitute onion rings for fries, and downgrade the drink to small, giving price P2 which is less than P1.
A third option is to ring it up as a small combo (Burger, small fries, small drink), upgrade to medium fries, and then substitute onion rings for fries, giving price P3 which is less than p2.
Is the seller in violation of the law if the clerk does not figure out and use option three?
How about an order for multiple people but all on the same ticket? For example here in the US Arby's has gyros for a limited time. They are something like a little over $4 each, but they have a 2 for $6 deal. Suppose a family of 5 comes in. The husband orders, then each kid orders, then the wife orders and pays. Suppose 3 of the 5 people ordered a single gyro each. If this were in Australia, would the clerk be required to notice that he can put two of those gyros together and ring them up as 2 for $6?
In reality though, these laws are intended to prevent misleading price representations, not to force businesses to calculate the best deal for their customers.
If you're interested, the main underlying EU rules are in Directive 2011/83/EU.
The most degenerate example I'm aware of, is that my boss lives on a county border. On the other side of his street, sales taxes are 7.5%. His are 9%. The zip code, city (kind of - technically one side is part of another city, but the post office recognizes it as the same city), and street name is all the same, but the county is different, which you don't collect for shipping reasons.
Every online retailer I've seen collects it wrong, including Amazon, although maybe their backend for reporting purposes calculates it right. When searching for software solutions to sales tax, most of them seem to get it wrong too.
They do often ask for a post code at the checkout to calculate shipping, however.
In Australia the full price for a product has to be displayed equally or more prominently than any other price. So that $6.93 has to be just as visible other pricing claims (like $0.99 for 4 weeks).
The same law also requires companies to include tax prominently in their pricing. More generally, a business cannot engage in misleading or deceptive conduct.
The rationale behind this is it makes things fair for consumers, and helps businesses compete fairly.
These laws probably don't exist in places like the US as businesses complain about compliance costs, and the governments have other priorities.