If you start putting 'terms' in there then you will create doubt and their finger may move away from the buy button.
Using Clickbank? Then you have no choice because the mandatory guarantee is 60 days.
You also have to give a guarantee when using PayPal.
In my 'offline' business I give a product lifetime guarantee and I've never had any ask for their money back.
Another very good business is called Landsend (direct clothing supplier). Their guarantee is also lifetime. You can send goods back, no matter what the reason and they will be exchanged or refunded.
Once, my wife borrowed a jacket from me to do some painting outside. Yep, she got paint on it, bur hadn't told me that she'd borrowed it.
After a quick call to Landsend, the replaced the jacket free of charge.
I've been buying from that company now for about 10 years.
Consider the lifetime value of a customer with a top notch guarantee.
Serial refunders are very much in the minority and there are ways of blocking their IP address so that they cannot buy from you again.