Need help on applying a dropbox upload button for clients to use after purchase.

I understand that you cannot add this to a confirmation invoice but I need to find a way to make something similar to this happen. I have clients who purchase my service of writing a song for them. They purchase online through the wix shop and after the purchase I need them to receive either a confirmation ( not possible) or just an email that has instructions on what they need to do to upload their music and lyrics to me online. I am open eared to anyone who could maybe suggest an alternative to doing this or something similar. : )