I recommend you implement it just like this:
1. In your first "thank you" email autoresponder message, after someone purchases your product, let them know that over the next handful of weeks you'll be sending them a series of unadvertised bonuses. Then, let them know the first one will be coming in about 7 days.
2. 7 days later send them an email with the first of 3 unadvertised bonuses. Make sure all of your unadvertised bonuses are quality stuff. Stuff you'd feel good selling on their own. At the bottom of this email let them know that in about 2 weeks you'll be sending them their second unadvertised. Give them a little teaser of what they're going get.
3. 12 days later send them an email with the second of the 3 unadvertised bonuses. Again, at the bottom of the email let them know that in another 2 weeks you'll be sending them their third and final unadvertised bonus. Give them a little teaser and make sure they know that the final bonus is the best and most valuable of all three (make sure it is).
4. 14 days later (now a total of about 32 days after their purchase - and brining them beyond a 30-day guarantee period) send them an email with their final unadvertised bonus. In the email, give them a tease again of what they're going to get, then ask them to take 60-seconds to fill out a little survey to grab the third bonus.
Then, send them to a webpage with a few testimonial-generating questions (i.e. "how has X product helped you over the last month", "how much money has X product made you since buying it", "what would you say to someone thinking of buying x product", etc.). Once they fill out the little "survey", send them to the webpage to download their third bonus.
The reason why this system works so well is because it keeps customers engaged beyond the refund period, it taps into the law of reciprocity, and it exceeds their expectations.
The end result is that you'll drastically reduce refunds, generate some killer testimoinals, and develop long-term customers who know you over-deliver value.
I posted a video on my blog a little while back that walks you through the system if you want to see it in a "process map". You can check it out here: TheSneakPeekBlog.com