I am not sure how does the WP extension letter looks like maybe I can refer you to senior members @scylla @DelPiero07 @jes_ON to review and comment on.You mean Offer Letter ?
Well Letter I am using is the one my company have filed for Work Permit extension.
So, It says salary, job description, full time, etc.
with that being said, include your pay stubs(6 months), T4(redact your sin#) as well.