Hi Dee,
I was thinking of posting the same question - not for crossword entries, but to get my mail-order deliveries on their way. I am glad I didn`t, because although your question is perfectly clear, the two answers are the usual dick-head sort. i.e. Lets answer quick, we won`t read the question properly.
Like you, I could drive five miles to the main SORTING OFFICE for our area which has a posting-slot on it where presumably the letters are not taken out but go into the sorting system directly. Does this mean that post collected from boxes up to Saturday mid-day are all lumped together on Sunday night, sorted, and sent on their way?. I would think this is logical - just as you did.