This would decrease their profit per-item by 1/2.
Key piece tho, are you able to return pre-sorted mail to sender?
Frankly, junk mail is one of the primary things keeping the USPS afloat.
https://www.paperkarma.com/product/paperkarma-subscription/
> Mail Withheld From Delivery. An addressee may request his or her postmaster, in writing, to withhold from delivery for a period not exceeding 2 years any foreign letter or printed matter bearing a specified name or address appearing on the outside. Such mail is marked “Refused” by the Post Office™ and treated as undeliverable.
https://about.usps.com/what/business-services/delivery-growt...