Returns are the problem: returns may not get the nice discounts for shipping, they require manual review of the products (are it the correct SKU, the correct size, is it in a resellable condition, etc.), restocking and reinsertion into buyable inventory, etc. And then refund processing, which may take 7-10 days or more.
But it doesn't address the problem of renters: they won't buy a subscription box if there's a monthly fee associated with it. They like being able to get new shoes, clothes, and accessories to wear once and then return for free. Identifying renters and either directly contacting them to change their behavior (this is high touch, but the company culture encourages this) or removing them as customers are the best solutions I've seen to date.
The downside is that you buy them all and get a refund on return.