In Massachusetts, human pepper sprays are not legal, however animal repellents are.
Massachusetts defines ammo as regarding to "tear gas cartridges, chemical mace, or any other device or instrument which contains or emits a liquid, gas, powder or other substance designed to incapacitate". To carry, or sell "ammo", a license is needed. Residents of Massachusetts may only buy self defense sprays from actual licensed Firearm Dealers within that state.
To sum it up... You must purchase pepper spray from a local store, not online. HOWEVER, you can buy animal repellents such as dog or bear spray. See the products below, which are legal to buy online, and legal to use without any license.