[notyourwork]

You left out the rest of the sentence which makes it an incomplete sentence. Usually this is followed by "would you like another?" or something along the lines that becomes a yes or no question.

Merely saying "This is two for x." is a statement, not a question.

[kzrdude]

That's why it tripped me up, they didn't ask a question. I just mundanely said "Ok" to their statement and it was understood as an intention to buy

[syockit]

That's why I make it a point to say "I see", "uh-huh" instead of "OK" to such statements, given that it's so easy to be misunderstood even though technically "OK" is an acceptable response.