Is this the final piece from the old 1990s USA Sonic Birthday supplies?
It could be. This is a set of Thank You cards to send out to all of the guests after the party. However many invitations were in a pack (likely 8) there will be the same amount of 'thank yous' in this one. It's quite highly likely to be from the same set because of the purple background that everything shared. Well, it's here too, with an 'outer space' looking rectangle behind getting ready to run stock Sonic. The cards say "Thank U ...And Then Some!", with the logo in color at the bottom.
What's curious is that the "you" is abbriviated to "U" for no apparent reason? Nothing else does this, there was room for the text...so why start here?
Notice how the edge has colored random/tile squares along it. This seems to match the round 'mylar balloon' on this page, as well as maybe the pennant banner at the top? While CA Reed used mostly purple on its packaging and products. Hmmmm.
This could be the rarest/least common party item. (Being from the early 90s only getting seen in 2020)