Here is a collection of postcards released for Sonic Adventure 1 on the Dreamcast. These came out in 1998. In the collection, you got 1 card for each character in the game. Knuckles on a red card, Dr Eggman with his name across him on a black card with small text, Tails on a yellow one, E-102 Gamma on a gray card with his name down the edge, Amy on a pink card with her name at the top, and yes that's Big back there (you can only see his ear) on a blue/violet horizontal card. Sonic, and some text (it's way too small to read) is on a horizontal blue card that almost matches his color.
The card backs are nice too, with orange and black ink, Sonic face stamp-placement indicator, and SA1 watermark style logo. This is a nicely designed set of post cards. Were they a bonus item?

At the right are more Sonic Adventure 1 cards.
This may be a second set, or it's possible the set seen above on this page wasn't complete. This has close up portrait character cards, full-body picture cards & CG cards. It's fun to include the different kinds of special chao you could raise in the garden. There's the chaos one, the silver one, & 2 shiny mystery colors. It looks like Chaos 0 has a CG card as well, possibly. What are these cards for & where were they released/could be bought?