The answer lies in the language of the Mass. Auto Insurance Policy describing PIP. An auto insurer is required to pay for reasonable and necessary medical services that are related (or as a result of) to your Massachusetts auto accident.
The key to getting reimbursement, though, is in the “documentation.” The auto insurer is usually going to require good paperwork from you to substantiate the out of pocket charge. For example, the insurer may want you to produce a copy of a prescription from the medical provider and the receipt from the pharmacy or store where you bought the equipment.
See our blog post Is the Medicine Ball Prescribed By My Therapist Covered by My Massachusetts PIP Insurance?