I just ordered the book after seeing this aleatorick's post on the blog (probably as soon as he had posted it).
'just ordered' is usually used in the sense of 'I ordered it just a few seconds ago', which is not necessarily what you intend to convey. It would be better to move 'just' to a position where it will do more good, e.g.:
I ordered the book as soon as I read aleatorick's post on the blog (probably just after he posted it)
'as soon as I read' - i.e., you read the post and immediately ordered the book. "Read' is better than 'seeing' if you want to indicate that it was the contents of the post that caused you to order the book, not simply the mere existence of the post.
'just after he posted it' - past perfect is not really needed as it is quite clear from the context that the reading occurred right after the posting. It is best to use simple tenses where possible, and limit more complex tenses to situations where they are really needed.