This is a "Joypolis Original Letter Set"
But, what exactly does that mean? It's made for Sonic Adventure 1, and came out right around the time of the game. It uses the 'circle strange walking Sonic' graphic pretty much everywhere: front cover, back cover as line art and inside too. It has the 'no regrets / go own way' quote written around the graphic, and, interestingly uses texture to add the word "Sonic" in black on the black background.
The 'set' appears to be a folder of some kind, with two compartments for stuff. Was it stationary and envelopes? Post cards? Did it have stickers or notes or something?