It depends on how established you and and how established the retailer is. A lot of small retailers will take your shirts on consignment, meaning they won't pay you until they sell the shirts. After a set period of time they would return the unsold shirts back to you. Of course negotiation is key. But this method is usually the easiest to set up with little risk or upfront cost to the retailer which for a new t-shirt line will not be cash positive initially but will help get more shirts into more stores.