How are reference letters for postdocs submitted? I am right now beginning my first application for my first post-doctoral position. The announcement lists a series of documents which should be submitted, including (quite predictably) a reference letter from my supervisor.

How would such letter normally be delivered? I am asking because of a certain cognitive dissonance. On one hand, the application lists it just like another document, which would suggest that I should simply get it from my supervisor and send it together with all the remaining paperwork. On the other hand, I remember that when I was applying for my PhD, the process of submitting references was rather more complicated, and specifically excluded me having access any of the reference letters. So maybe I should ask him to post his opinion directly to where I'm applying. How do these things typically work? (I imagine that if no clarification is given, there is a universally agreed way...)

If it matters, I’m now in UK, and applying for a position in Eastern Europe. I study pure mathematics.
Postdocs in mathematics in the US are universally* handled through MathJobs, which was commissioned by the American Mathematical Society for this purpose. Recommenders submit a single letter, which can then be attached by the applicant to any number of applications, without ever giving the applicant access to the letter. Recommenders can also send a more specific letter to particular schools if desired.

This system has been so successful that it has been made available to other fields as AcademicJobsOnline by the Duke University Mathematics Department, who originally created MathJobs.

I do not know whether positions in Eastern Europe use MathJobs, however. 

*If anyone knows of a postdoc in math in the US that does not use MathJobs, I'd be interested to know; it's been a few years since I last heard of one.