This is a St. Vincent OHMS, or ON HIS/HER MAJESTY'S SERVICE Cover.  This one was sent from the Colonial Postmaster to Boston, MA on 11/12/1909 via Registered Mail.

What makes this little beauty really special is the RARE Registration Label (see photos) & the use of the block of 4 of the St. Vincent Queen Victoria SG 63, the THREE PENCE on 1d Mauve.  These are valued at L120 x 8 on cover.  It is also backstamped.

This item is over $100 USD so I will ship it using Priority Mail in the US, and Registered Mail overseas.  This is to ensure that it reaches you in a safe and timely manner!