Not having to carry or interact with a physical key is the new hip thing. Cars have had keyless entry and push-button start for years, and now Teslas can even be used with just a phone app. Smart locks for homes have been around for some time as well, and if you don't already have one, you might want to look into this deal on the August Smart Lock 2nd gen, which is now being offered for $105.79 via eBay.
This is basically the best price we've seen for the second-gen August Smart Lock thus far, with a $99 in-store Walmart deal we posted about last month not being very accessible for many. The unit installs over your door's existing deadbolt and only replaces the inside portion, with the exterior still keeping the ability to be locked and unlocked via a regular key. It uses Bluetooth, though as we previously mentioned, you might want to pick up the separate $60-ish August Wi-Fi Bridge for remote unlocking, Home and Alexa support, and a better experience overall.
If you're interested, head over to the eBay listing. The seller in question has a ton of feedback and a 99.5% rating, so you shouldn't have any issues. Free shipping is a nice touch.