If one does volunteer work for an organisation and recieves a shirt as a complimentary gift. By wearing the shirt in public people would know that that person volentered. would this in any way reduce the level of chesed that was done.
Organizations give out sweatshirts to volunteers to make them feel appreciated, as well as to publicize the good work of thye organization and to encourage others to contribute and volunteer themselves. Consider wearing the sweatshirt as part of the volunteer activity that benifits the organization.