That's an interesting one. A private parcel post delivery company formed in 1898 (corporation dissolved in 1904) in Boston, Massachusetts.
The stamp is apparently from 1899 according to a listing by Eric Jackson. It is said to be identified in the Mosher Catalog as PPCD-S1.
Very little appears online about the company other than the corporation being established (1898) and later dissolved (1904). However, I did find this newspaper article interesting about the delivery equipment they put into use: