A 12 percent return rate sounds modest until you price it. Take a retailer shipping 10,000 orders a month at an average basket of 70 euros: 1,200 returns, each with a label, a parcel coming back, twenty minutes of inspection and restocking, and a refund. At typical European rates that is 9 to 14 euros per return, so 11,000 to 17,000 euros a month before you count the goods you can no longer sell at full price.
The cost splits into three parts you can act on separately. Transport is negotiated with carriers, and return labels can be priced apart from outbound. Handling depends on how clear your inspection rules are. Lost value depends on speed: a garment back on the shelf in three days sells at full price, one back in three weeks goes to the outlet. Measure all three before you touch the policy.