No, usually 'leftovers' is food, but you can say 'leftover', such as 'leftover product lines'.
Your sentence is not very natural. Perhaps:
Even though we have sent several deliveries out over the past few months, there are still quite a few leftover shoes.
However - 'leftover' has the connotation of unwanted. I would not use that word to a customer - they would probably expect to get a discount! ;)