Based on the user interviews, I outlined the following key features.
Key feature #1
A rewards point system is a necessary motivator. One of the people I interviewed indicated they would keep their clothing in their home forever, and would only consider donating or recycling if there was a reward or incentive for doing so. They further indicated that a rewards point program that resulted in discounted new purchases was a plausible solution.
Key feature #2
Access to a map that displays the drop-off point nearest you and their operation time. Most people prefer to find a drop off point near them and bring the items there on their own time, and without much hassle or distance.
Key feature #3
For some people it would be easier—and in cases of the elderly or differently-abled, necessary—to provide an at-home pick-up feature for their old clothing items. In this scenario, we would need to design and implement a calendar where users can select the date and the time that suits them.
Further considerations
To make it simple for both users and businesses, people can drop-off their items at the store and instantly earn reward points by scanning a membership card at the point of purchase (many clothing brands have a membership card that customers can get for free to collect points and receive benefits/discounts already).
A pick-up service poses further complications, since the person who collects the used items would need a reader and a face-to-face interaction to scan the user's card. If it's not possible to have face-to-face interaction there must be an option to print out a bar code and attach that to the package in order to collect the points.
For the purpose of this design challenge, I am going to focus on Feature #1 and #2.