In formal letters, I don’t use “s’il vous plait” nor any abbreviate form. “S’il vous plait” is in my opinion more frequently used in oral conversations, for example when you buy bread. Possibly in an e-mail…

If you’re going to write some form of it, use “s’il vous plaît ”. Abbreviating it would appear like authoritative rudeness in full text (unlike in formulas like “Tournez SVP” near the bottom of the page, for instance).

Now, depending on how you phrase it, it can easily seem you’re begging for someone’s will, which may not be what you want to express. Alternatives could go :

• Pourriez-vous …
• Je vous serai(s) obligé de bien vouloir …
• Veuillez … [, s’il vous plaît]

depending on how formal it is, and especially on the balance of powers.