I'm in the UK, and ASOS does this already. My partner regular orders the same clothes in multiple sizes, and sends back the ones she doesn't want in the bag they're sent in, with the prepaid return label.
Adding to the UK perspective, the following companies all do it as well and are representative of both older and younger shoppers:
* Boohoo
* Pretty Little Thing
* Missguided
* Marks & Spencers (non-food)
* House of Fraser (under 20Kg)
* Debenhams
Outside of these, with "Collect Plus" now being next-to ubiquitous in the UK (over 6,000 participating stores) any store using Collect Plus (or DPDs Ship-to-shop) can offer free returns via the stores. As an example of how useful this is in general, the tiny corner shop on my street is a Collect Plus collection point.
Another one for the UK, Doddle provides pick up locations at train stations and other places designed for commuters.
They even have places to try on clothes, and packaging materials there, so you can return the items without ever taking them home.
Unfortunately they are quite expensive from a retailers perspective, or roughly equivalent to a Prime membership if the user subscribes directly (so they can use it with all online stores).
Customers must pay an absolute fortune on returns (embedded in cost of clothing), instead of that money generally going to paying landlords - who constantly put their rent/leases higher.
I feel like your partner should have to pay extra for this service. As it stands the cost of all this extra shipping is built into the base price. Everyone else is subsidizing your partner's behavior. I wonder what percentage of customers do this.
Walk into a post office or collect plus in the UK and look behind the counter. They're all rammed full of asos returns. She is not alone in this behaviour.
There are subscription services that are designed specifically around this behavior. My sister in law uses one I think called stitch fix. You give them your measurements, interview, and then they assign you a fashion consultant who picks out clothes for you and sends them on a regular basis. You send back whatever you don't like. It must be pretty profitable if more traditional retailers are heading partially in that direction.
