Ring Rechargeable Battery Pack for Video Doorbell 2 and Spotlight – $20 at NFM (free shipping at $49+)

Link: Ring Rechargeable Battery Pack for Doorbell 2 and Spotlight
This is a YMMV because NFM doesn’t ship everywhere and you need to place an order of $49+ for free shipping, but if you need a few of these, you can’t beat the $20 price.