how to send automated emails when an user has finished payment

I am currently making an online store in html and css about 3d models and stuff. I have set up paypal buttons like add to cart and buy now buttons. I would like to do something like when the user has completed the payment, an automated mail sends him/her to with the download link. Just to test stuff i have set up an freenom domain with an gmail business mail through some youtube tutorials. Paypal offers like when the user has completed the payment, paypal will redirect him/her to a different webpage. The user will then can download the stuff from the mail. And also like they have to put their email id in order to place order, It is pretty easy with some knowledge php and database and stuff. Thank You