Give the clients the option to pay to extend their proof gallery. Like a republish fee to put a gallery back online for an additional day/week/months... It'd be great to have that option automated as their proof is about to expire. Client can pay via credit card and extend their link to their needs.
Similar to Josie I added this as a digital product - unfortunately I found out later that my client was charged sales tax (illegal in my state) and the only workaround is to add a 'single dowloand digital file' to accompany it - which is extremely confusing for clients.
I've done something similar to this, Just create the item in the products but you would have to manually change the expiration date on the gallery once it was purchased.
I suppose you want an automated version though.