Spigen Inc. via Amazon offers the Spigen Most Durable Rounded Edge Glass Screen Protector for iPhone 6s, model no. SGP11588, or iPhone 6s Plus, model no. SGP11634, for $7.99. Coupon code "XNBULAVW" cuts it to $1.60. With free shipping, that's $23 off list and the lowest total price we could find. This 0.4mm protector features a 9H screen hardness and oleophobic coating to resist oil and fingerprints.

Of note, the above coupon may work on other select Spigen items for iPhone 6s or iPhone 6s Plus.