You can do the same with GitLab as another option, it supports custom domains too.

I seem to get pop-up notifications for free in GNOME/Fedora by setting these levels in /etc/UPower/UPower.conf:

UsePercentageForPolicy=true
PercentageLow=50
PercentageCritical=20
PercentageAction=10

I think you can also configure the system to take action when it reaches the lowest level with e.g.

# The action to take when "TimeAction" or "PercentageAction" above has been
# reached for the batteries (UPS or laptop batteries) supplying the computer
CriticalPowerAction=PowerOff

However I don’t know how to get these GNOME “Power” notifications to play an audible sound (without turning on notification sounds for ALL notifications). The best I could find is this: David Bazile / gaudible · GitLab

There’s talk of better control of sound notifications in GNOME 47+, but looks like nothing much has landed yet: Notifications in 46 and beyond – GNOME Shell & Mutter