Newegg offers the Asus Dual-Band Wireless USB 3.0 Adapter, model no. USB-AC55, for $19.99 via the steps below. With free shipping, that's the lowest total price we could find by $10. It features WEP, WPA, and WPA2 encryption. Deal ends August 31. To get this deal:

1. View the Asus Dual-Band Wireless USB 3.0 Adapter for $49.99
2. apply coupon code "EMCAWKW49" to drop it to $39.99
3. redeem this $20 mail-in rebate for a final price of $19.99