LightInTheBox offers the VR Box Virtual Reality Headset For Phones for $12.99. Coupon code "US2OFF" drops it to $10. Plus, select no tracking to bag free shipping. That's $8 under our June mention and $7 under the lowest total price we could find for a similar headset elsewhere. It fits most smartphones from 4.7" to 6".
Note: Uncheck "Add shipping insurance" to avoid the 49-cent fee.