How do I write an email to sell a product sample?

First of all, never ever try to sell anything to anyone. Because there is a human psychology, no one likes to be sold.

So, always try to educate them by adding the values in their current process.

I try to elaborate it by some examples:

If you are selling a credit card, focus on how it can be helpful in saving more money. Here, don't even mention “purchase” or “take it as soon as possible” etc, type pushy phrases. Just focus on the enlightening. Once your prospects will get to know the value of your credit card, purchase/sample sale will automatically be taken place.

[Above is a real life example. I sold about 90 cards (by cold calling and emailing) in a month.]

Second, a B2B product:

Let's say you are selling an account solution. Focus on how your product can easily perform the most critical and time-consuming tasks. Find out the jobs which take more time and are hard to complete, then approach your prospects with the solutions. Again, I would say, don't push for sale. Just educate them.