That sheep gene is kicking in again Boyo.

All the above can possibly mean is the
advertising of something like a cheap chocolate bar on a poster.

Er.....no it can't.....

I believe that Amused is using 'poster' as the noun form of the verb 'to post' and not in the sense of a billboard poster/large piece of paper containing advertising.

Unless I'm wrong, it seems that you are one to have been 'taken in' since you appear to have totally misunderstood her context.