UPS has a station at AirVenture during the fly-in. You can ship from Clinton, but it will won't go out until Monday, and arrive OSH on Tuesday if you ship overnight.
There is another way though. I'm driving to both events, and I have room.
Hi Mark! Well that certainly sounds like a generous offer! I plan to work out my items and weight for safe flight for me and grandson Trevor. That will certainly involve shipping some stuff to Clinton. I did reach out to EAA and they confirmed (as you already stated) I can ship in and out per info found at this link:
https://www.eaa.org/airventure/plan-your-eaa-airventure-trip/guest-services/shipping-services. However, as I hope to fly up to Oshkosh on the Sunday it would be great to be able to get my camping gear there in time for that first overnight. So I hope that I might be able to take you up on your generosity!

See you there!