Enable download file after payment in ASP.Net

I have a 3 files in the pdf format for sale. I displayed these files in the Grdview like that

ID       File Name      ContentType     Date             Price      Action

1             ABC                 .pdf                              500               Download

2             SKY                  .pdf                             700                Download

3             XYZ                  .pdf                               800              Download

Requirements

When any User in the internet comes to my website and he wants to download the file then

1- Any User should be enable to download the pdf file autometically only when he paid the online Payment for the required files.

2- Any User cannot download the any files until he pay the required payments. how to get the solution plz