Here are 4 postcards by GE Entertainment.
Theres' a card for Tails, Sonic, Amy & Knuckles. They're all done in the SA style with their name written in the logo font at the bottom. Each card has the characters' portrait on the front and the usual divided back (half for address/stamp, half for writing) These cards were sold as a pack, with several of each one. They're nicely designed, and would even look cool pinned up on the wall or as the background for a shelf.
Somehow, these weren't very common. They're likely a 201? (2010 or later) item, but it hasn't been pinned down where they were sold. This photo of them is actually from Japan because someone found a way to import them. How many cards did you get? How much did it cost?