Amazon also offers its Prime members the Logitech M570 Wireless Trackball, model no. 910-001799, for $27.49 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, it costs an extra $8.58. Walmart also has it for the same via in-store pickup.) That's the best price we could find by a buck, although we saw it for $7 less in December. Features include programmable buttons, 2.4GHz wireless, and up to 18 months of battery life.