[HELP] Additional Documentation is Required for Steam Community Market

Hello guys, Merry Christmas.

I was trying to sell my leftover trading cards on the community market when I reached the 200 items limit when I recieved a message from Steam about my non-US status.

This is the email:

Thank you for submitting a Form W-8BEN to certify your status as a non-U.S. person. We have identified an issue with your certification that will require additional documentation to cure.

Even though you may never have been to the U.S. or lived in the U.S., you have previously used a U.S. address in one or more failed or successful purchases on Steam. The most recent transaction reflecting a U.S. address can be viewed on your account page at https://store.steampowered.com/account/ underneath the "Tax Information" section on the right side of the page. Because you have used or attempted to use a U.S. address on Steam, you must provide ALL of the following:

One piece of legible and unexpired documentation indicating foreign residence. The name on the documentation needs to match the name used to complete the Form W-8BEN. Acceptable documents include copies of the following: Driver's license Passport Residence card National identity Card Other government issued photo-id Birth Certificate (only acceptable for persons under the age of 18) AND A written explanation to substantiate foreign status. The explanation must meet these criteria: Must be reasonable and explains why you used a U.S. address on Steam Must support your foreign status Must not indicate that you are a U.S. person Examples include, but are not limited to, the following: Selecting "United States" as your country by mistake during checkout Entering a U.S. address while visiting a friend or relative on vacation Using a credit card that belongs to a family member who resides in the United States AND Your response must include your seller ID. Your seller ID is: xxxxxxxx

Please note that Valve uses a third-party validation service to receive and validate this additional documentation.
All three pieces of information as indicated above must be sent either by email to: valvesoftware@cticompliance.com, or by mail to: PO Box 1635 Andover, MA 01810.

Said transaction was made back in 2012, obviously I can't remember why I did it back then. Maybe there was a price glitch or something.
Can you help me with a good explanation regarding the matter, as I'm not into this stuff.
Sorry for my English and thank you in advance.