There's obviously no way to ensure a sale through, but to increase the chance of sale through as much as possible would involve using a combination of several known marketing/psychological techniques. Some of these would require for you to have some control over the text of the mail sent. However, if you are not allowed to change the text in any way, there are others techniques that you could couple with their mail to enhance sale through.

A discussion of which techniques to use, and how, would require further knowledge of your limitations, and the type of mail you're sending. If you'd like to discuss your options in relation to your specific problem let me know, I'm be happy to help,

all the best,

Lee