The instructions about how to subscribe to Logbook of the World (LotW), by the US Amateur Radio Relay League (ARRL), are a little confusing and somehow contradictory, depending on where exactly you retrieve them. ARRL website says one thing, "Trusted QSL" (TQSL) software says another thing, on the several radio amateur web sites we can read interesting tails but, the most amazing stories could be found in the several hamradio forums and discussion groups. I subscribed to LotW in May 2018 as non-US radio amateur; I just downloaded TQSL and installed it. Then...
Then, as I was saying, instead of sending the paper letterthrough the international postal service, I scanned the CEPT license and emailed the pdf file to ARRL. The day after, I've received by email both the file with the key to sign the logbook files and the password to use the service. Therefore, I would say that the story about the need of a paper letter as unique way to subscribe to LotW is just a legend. Of course, one could subscribe by just sending an e-mail message, thus saving a lot of time and even a little money.
I cut and paste here below the text of the e-mail that I received, where I have removed some personal piece of information. 
Da: lotw-admin @ arrl.org
Inviato: giovedì 31 maggio 2018 19:30
A: xxxxx @ xxxxx.xx
Oggetto: LoTW Certificate
Allegati: IK1XXX.tq6
IMPORTANT NOTE: This certificate requires TrustedQSL Version 2.0 or later.
Attempting to install it using an earlier version of the TrustedQSL software will result in errors. The latest TrustedQSL software can be obtained at:
https://lotw.arrl.org/lotw-help/certaccept/
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Here is your LoTW certificate for IK1HGE You should be able to install this certificate by double- clicking on the attachment icon. If that isn't possible or doesn't work, save the attached file to disk and then use the TQSL program's "Load Certificate File" menu command to install this certificate into your system.
NOTE: If the attachment failed to arrive with this message, you can just log on to the Web site noted below using the provided username and password and download the certificate file directly.
Records submitted using this certificate can be accessed on the LoTW web site using the credentials:
username: xxxxx
password: xxxxx
by logging in at <https://lotw.arrl.org/lotwuser/default>
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NOTE! This is NOT the same password the TQSL program may ask for when you're signing a log file!"
