Austria - USA economy route doesn’t seem to work, or works very very slowly, over 100 days.
But: priority seems to work, and apparently you can use stamps.
Priority mail should be taken to post office and have priority label.
Please update this, so Austrian members could use this option if they are willing to pay more, and have more quick delivery. (Although long time members probably won’t check this.)
Here is my suggestion:
Attention: to USA, priority option seems to be much quicker while economy seems to be very slow (priority about two weeks vs economy over 100 days).
To other countries economy still seems to be a quick choice. (?)
Priority should be sent from the post office. You can use stamps, and the sending must have a priority label (you can print that or get from (some) post office(s)). If you drop it to the mail box on the street, it is not necessarily noticed and can be treated as economy mail.)
There is a thread about this: