How box links work
Each box has one private link. Send it to your client. Anyone with the link can see the box, add files and notes while it is open, and download its files, so share it only with your client.
The link is shown once, when you make the box. If you lose it, or it reached the wrong person, use "New client link" in the box's More menu. The old link stops working at once.
Closing a box stops new uploads and frees its slot. Your client can still see and download the files during the hold period.