Sending Trial Ending Notifications with Pro Sites

Okay so Pro Sites sends an email to the user once their free trial is over. This is great. However, I'd like to be able to send multiple emails. My free trial is 14 days long, so I'd like to send them reminder emails at 1 week left, 4 days left, and 1 day left.

How do I go about doing this?

I know Pro Sites doesn't offer this out of the box, but I'd love some code to add to Pro Sites that adds this functionality. I'm not a coder myself, but I'm comfortable working with code and getting my hands dirty.

Any help here will be much appreciated.