One question about Sonic Superstars that has been lingering for awhile has been whether or not people with a physical copy will be able to get the Digital Deluxe content. Well, wonder no longer: you can! But it'll cost you.
Now that the game has launched, its DLC can be purchased, including a Digital Deluxe Upgrade. This "upgrade" will cost $14.99, and will include the rabbit skin, the Mecha Sonic skin for Battle Mode, the mini OST, the digital art book, and the LEGO Fun Pack. It's available on all platforms, and available to anyone who didn't buy the Digital Deluxe edition.
In addition to this, the comic skins are available on Steam for anyone who's purchased the game, and the modern Amy skin is also available on the store for $1.49. You can find them on the game's store page here, although you can currently still get the modern Amy skin via the newsletter for the next few days.