I do know that Digital Custom has trouble restoring photos scanned through glass. The last time we had one of these, we came up with a solution that worked for the customer and for us. She gave her permission for us to do the following:
First we scanned through the glass.
Then we gingerly removed the photo from the glass, knowing that a half inch would separate at the bottom and remain on the glass. Remember, this was OK with the customer.
Then we scanned the bulk of the photo that we were able to remove.
Then we sent both images to Digital Custom. They were able to use the image scanned through glass as a guide to recreate the missing pieces of the photo.
It came out perfectly.
I would not do this with a valuable historic photo, but for an old family photo where a good reproduction was desired, this solution worked well.