$50 for Amazon Key edition KwikSet conversion kit. Integrates your existing lock to ST via Zigbee, no need to involve the Amazon Key program (unless you want to).
https://tools.woot.com/plus/amazon-key-edition-smart-locks-2?ref=w_cnt_odet_w_crum_2_wp
Several other smart lock options also on sale here:
https://tools.woot.com/plus/amazon-key-edition-smart-locks-2?ref=w_cnt_odet_w_crum_2_wp