Why not offer a 90-day money back guarantee? You have a new, unproven product, and I would guess many people are on the fence, waiting for others to be the guinea pigs. A 90-day guarantee would show you have faith and confidence in the ability of this program to increase search engine referrals, and increase revenue.

If after 90-days the purchaser doesn't feel they've gotten their money's worth, they can uninstall the program, and request a refund. You deactivate the license, and refund the purchase price. Unless time is spent on support, it really costs nothing, and potentially greatly increases the installed userbase. After you get a decent base of sites using the software, and some success stories, then you can reconsider if it's needed.

Because they trust their script %100

It's a nice concept sure, but the problem is people abuse these sorts of things.

99% of the time in the online software industry, once you've downloaded the software, there are no refunds.

Okay. However, I'm not sure how they'd abuse it if you have an encoded script that checks the license status.