It's not a law that candidates use union, it's that they want union endorsement, so they buy union products. You didn't say where you are. But if there are a lot of union shops in your city, then breaking into this market would be hard. They just don't all flock to you just because you are union.
Fulfillment companies basically allow you to have an ecommerce solution with little or no costs. You upload your designs to a website they provide just for you, then people buy products with your designs on them. The company then fulfills the order by printing and shipping the product directly to the customer.
Your post probably fits better in the T-Shirt Industry section.