Sage doorbell sensors have finally found their way to eBay. I bought a 2 pack and tried it out, and after verifying they worked I wanted to mention it to the community. I am in no way affiliated with any of the sellers, but here is one of the better deals:
A box of 12 for $15, but they will take offers for less. I know because I bought a pack for no good reason besides maybe some other project.
I used this driver:
And created a Pushover notification through RM.