What is the Sie doing here? Since it's in the imperative, why couldn't this be "Bitte verschicken ihn noch nicht"?
"Sie" is always used in polite imperative. If you address a person with "Sie" (not with "du"), you have to use "Sie" in your request: Helfen Sie, Gehen Sie, Wollen Sie mir bitte sagen, etc.
If you are on informal terms (i.e., you use "du"), the imperative is formed with the verb without ending and without a pronound: Hilf, Gehe, Sag mir bitte, etc.
The focus of 'verschicken' is on the act of sending. So you'd say 'Wir müssen noch die Briefe verschicken' (meaning we have to put stamps on them and bring them to the post office) but 'ich schicke dir einen Brief'.