Great job Barbara!

Your entry was completely comprehensible, and you've even used some very advanced grammar points (especially with the verb tenses, those are tricky even for natives!) In yellow you'll find some grammar corrections, in green are more stylistic corrections - they're not really wrong, but just a liiitle weird c:

This is a topic that I truly love. I’ve been meaning to write something related to this type of art for so long, that I don’t even know where to start. Mail art refers to an exchange of letters, envelopes, stamps and any other kind of little gifts that you might want to send. In my case, everything it all began some a few years ago with a postcard from Seattle. The postcard was nothing special just the typical greeting, but for me it was so meaningful that had to reply to it, and since then, a beautiful correspondence between my friend and I started. The exchange lasted 9 months, in during which the main means of communication was were the letters. It was such marvelous time, that I will keep in my heart those memories forever. (albeit technically correct, it sounds a little weird. Personally, I'd write something like "I will cherish those memories forever") Coincidentally, at that time my friend had a strong fascination with everything related to fountain pens, inks, and papers, so it was easy for him to get engaged in this game. I remember that my envelopes used to be quite a bit girlish, with a lot of flowers, glitters, and pink everywhere. I also used to add a lot of little gifts that I knew my friend would be happy to get. But the most amusing part for me was when it was time to choose the stamp! I just loved to spend spending a long lot time at the post office choosing and buying stamps. At the same time, as since my friend was aware of my obsession with stamps he used to spoil me by letting me choose the stamps that I wanted him to buy for me directly from the website of the USA post office. (again, not technically incorrect, but I'd write "USPS's website") In summary, all these tiny details are what make is what makes (here, you're referring to "all" which is a group word and corresponds with singular "is")Art Mail beautiful. Is It's not only the letter itself but also the love, time and dedication that you put into each envelope. What happened after those 9 months? Well, we finally saw each other face to face for the first time. Can you imagine what is it like to write to someone daily, for 9 months without ever having seen not even a single picture of that person before?? ;-) 

I hope your rendezvous turned out marvelous, and best of luck with your learning! c: