Clients Deleting Pre-paid Collections/items In Carts

I have started letting people do "Pre-Paid" collections/items for team & individual sports photos.  If they do pre-paid, they get a discount (that's their incentive).  Once they pay, it leaves the cart active within their account so that they can go back and load up the collection with the images/poses they want.  However, I have had a few instances where someone will delete the pre-paid collections/item from their cart when they go back to select images.  They then add the images/poses they want like normal and end many end up paying a second time.  Obviously, this is problematic if I don't happen to catch them.  I usually just ping the buyer and manually intervene to fix the issue.  Is there some way to prevent the user from deleting a pre-paid collections/items?  

The customer does not have the option to delete pre-purchased items. They can only be deleted from the cart from the admin. 

Maybe they are not logging back into the account they used to pre-order.

Can they "check out" without filling all the spots in a collection?  And if so, do those spots stay active?  Maybe that's what they're doing.  I have a person I'm looking at right now who should have had two orders each with two products (4 items to fill).  She filled three of them and checked out, but I don't see a way to manually go in and fill the fourth for her.  It still shows 3 of 4 filled on the orders summary pages though.  

Tim, I will create an admin account for you and you can check it out at your convenience.

Thanks!!!

If the collection they pre-ordered doesn't require a selection for each product, then they can checkout without selecting photos for those products.