How are reference letters for postdocs submitted?
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.