Tell me about trade routes

[Stylesjl]

I have a hard time fathoming how trade route yeilds are calculated in this game.

From what I seem to understand

Trade routes usually form between foreign cities because foreign trade routes yeild more gold. But:

Trade routes between vassal and master and inside permanent allies do not count as 'Foreign' trade. Therefore, no foreign bonus and mercantalism doesn't affect the routes

I also can't understand the sustained peace bonus, is that when the particular city or civilization hasn't faced any kind of combat or declaration of war?

Also, is the customs house useless under mercantalism?

So am I right about what I have said? Or have I completely gotten it wrong?

 

[King Flevance]

I also can't understand the sustained peace bonus, is that when the particular city or civilization hasn't faced any kind of combat or declaration of war?

I don't know for sure but I assume this means only between you and the target. BUt I could easily be wrong on this. I haven't looked up much on the subject.

Also, is the customs house useless under mercantalism?

Pretty confident this is a yes.

I am curious to see the answers to the first question.