How to send my customer free digital product automatically after a set amount of purchase...by using squarespace email automation or any other method..

Hi Awesome community!

I want to send my customer a bonus free digital file when they buy a certain amount of stuff... (Digital goods). Is there any way? I know i can set automated email through squarespace email campaign and syncing the digital file in the email through gooogle drive or dropbox..... But I am unable to set the amount of purchase for this automation. 

Please help, Will really appreciate.