YES, you can use for word “lifetime” in sales letter but we advice that you make it very clear for customers what lifetime means in your offer especially for warranty and guarantee part.
For example if “lifetime” in your offer means life of your product we advice to make it very clear for consumer what it means and what is warranty / guarantee consumer get.
You must always follow all FTC guidelines.
“The Guides advise that to avoid confusing consumers about the duration of a "lifetime" warranty or guarantee, ads should tell consumers which "life" measures the warranty's duration In that way, consumers will know which meaning of the term "lifetime" you intend."
Here is link to related part of FTC website:
This text is for informational purposes only and does not provide legal advice.