Once a contract is signed, the creator's verified shipping address is available in your campaign's shipment overview, so you can ship the same day. Creators verify the address in their own profile; if one is missing or looks outdated, message the creator via the 1 inbox to confirm before you ship.
You can also message a creator without leaving the shipment overview. Hover their row, click the message icon, write your message and press Enter to send it (Shift + Return adds a new line). It goes into your campaign chat with that creator in the 1 inbox, and Jump to conversation opens that chat. Your role needs permission to send messages; without it, you are asked to request access.