The story of Rip Van Winkle may seem incredible to many, but nevertheless I give it my full belief, for I know the vicinity of our old Dutch settlements to have been very subject to marvellous events and appearances. Indeed, I have heard many stranger stories than this, in the villages along the Hudson; all of which were too well authenticated to admit of a doubt. I have even talked with Rip Van Winkle myself, who, when I last saw him, was a very venerable old man, and so perfectly rational and consistent on every other point, that I think no conscientious person could refuse to take this into the bargain; nay, I have seen a certificate on the subject taken before a country justice and signed with cross, in the justice’s own handwriting. The story, therefore, is beyond the possibility of doubt.
This is from the short story, Rip Van Winkle by Washington Irving. Here, the narrator is trying to say what he previously told is absolutely true, there's no room for doubt. (though it's actually not. it's fiction.)
Now I get the overall point of the paragraph, but don't understand what the bolded parts mean exactly, especially the meaning of "take this into the bargain" and the whole of the following sentence.
I've looked up the meaning of "into the bargain" and still not sure if the definition fits here or if it has other meaning. As for the next sentence, I have no idea even about the structure. Is it "I have seen a certificate [on the subject (taken before a country justice and signed with cross)]"? What does "taken before a country justice" mean(I know the individual meaning of these words but can't make any sense all put together.)? Also, what's "cross" and "justice's own handwriting"?
Thanks in advance.