Hello Danilo,
Thank you for posting images from your Crete cover.
The one drachma stamp was originally issued in 1905 in the set shown in perf12's post.
This version with the large ELLAS overprint was issued in 1910.
These stamps with the large overprint were the last regular postage stamps to be issued by Crete.
The charge for registration was 25 lepta.
The international letter rate was 25 lepta for up to 20g plus 15 lepta for each additional 20g above this, so the weight steps would be 25 lepta, 40 lepta, 55 lepta etc.
I believe the cover is philatelic given the high weight that would require such postage.
The postmark is the original XANIA type which shows the time the mail was picked up.
I believe around 16 different times are recorded. In this case it looks like one o'clock in the morning.
