It is much of a muchness. I haven't studied contract law in about 20 years, so I'm going off my memory of the Carbolic Smoke Ball and Boots Cash Chemist-type reasoning.
In practical terms, the offer has to be put forward by someone. The buyer is unlikely to be offering to buy it 'at any price', notably because they're under the likely impression that it's on sale for $8.55 and because people realistically don't seek to buy something 'no matter what the price is' as a general rule (even though that price may feel like it).
The $8.60 is unknown to the buyer until it's communicated by the seller. In their mind, they think they're buying it for $8.55. By basic principles, it becomes a counter-offer.