Worthing mayor Alex Harman will be among guests at the Chestnut Tree House shop in Goring on Thursday, April 12.
The party will run from 2.30pm to 4.30pm and includes free refreshments and a birthday cake.
Jan Harper, head of retail, said: “Since opening last year, the Goring Road shop team have processed and sold over 25,000 items that were generously donated by local residents. A quarter of these items were gift aided, meaning an extra 25p for every £1 pound donated.
“In the last year, the shop has raised enough funds to allow 38 children to spend a night at Chestnut Tree House, where they will receive specialist care and symptom management from our nursing team and be able to enjoy the facilities which the hospice offers.
“We’ve been thrilled with the generous support from customers and donors since opening, and this party is our way to say thank you. We hope that all of our customers and supporters will join us on the day and share in the fun.”