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! ;)

I would say "unsold stock".