I'm reading Patrick O'Brians novels and I haven't figured out how post from Captain Aubrey's wife, for example, is forwarded to a specific port where the Captain finds himself.

Considering, in the novel The Surgeon's Mate, Aubrey escaped from Boston and landed in Halifax, why would he expect a whole backlog of posts to find him there as soon as he arrived?

I would think that it would harm the war effort if the post was going to be aware of where and when each ship would be landing and, being at war and also subject to the whimsical nature of weather, how reliable would it be to expect to see any ship land at Port at anything close to a set date?

I'd also be interested to know how banking worked during those times where one could go to a "local" branch and withdraw money and how those transactions crisscrossed great distances. Since post could be lost at sea what backup plans did they have to ensure withdrawals and deposits were correctly recorded?