Free Value to --> Sale (Prospecting)

I am currently sending direct mail letters out to some businesses in order to try and get a phone appointment with a decision maker to get a sale for a website re-design. I feel as if we have a pretty good USP which is:

If we don't make make you a re-design that will "convert" or get a higher response than your current website does with your current traffic then you get a 100% money back guarantee.

I am planning on sending out 300 total letters to different businesses.

In which I will test 3 different "methods" [100 direct mail pieces each] to see what will get the best response.

One method will just be the straight 100 letters.

The other two, I want to provide some sort of value upfront. The thing I am struggling with is how much & how can I really get them to want to schedule a phone appointment about our services after seeing the value we have given them up-front.

I was thinking something along the lines of sending them the sales letter along with a page that has something like,

"Top 3 changes to make to your site immediately to, XYZ"

Is that something you think would work? Should I be providing more value and/or how can I use that free value to get them to want more (AKA, pay for our services)?

Would it make sense to write something along the lines of,

"If you would make these 3 changes to your website today you could see up to xxx% increase in sales through your website. Schedule an appointment with us today and find out how we can increase your sales by xxx% with a site re-design." - and then sell them on the appointment.

I am just spit-balling here so let me know what you guys think, thanks.

PS- I have a side question for whoever wants to answer: I will most likely be sending a total of 5 pages or so in a small #10 envelope. I want to save some money, does it matter if I instead just printed the sales pages back to back on a piece of paper so instead I only have 3 total pieces of paper to send instead of 5?