2019 MacBook Air Stop working keyboard and trackpad not working

Not much to go on with your post. You also didn't give us any clues on what Apple found that would require a 500 repair cost.

I suggest that you try the troubleshooting steps in the following user tip article and let us know how it goes:

• Mac Basic Startup Troubleshooting Steps - Apple Community

Jmitch1980 Said:

"2019 MacBook Air Stop working keyboard and trackpad not working: Help my 2019 MacBook Air keyboard and trackpad not working. My warrant is out and Apple wants 500 to fix it."

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Getting the Keyboard and Touchpad to Work:

Reset the SMC and NVRAM Using an External Keyboard:

Sometimes changes(i.e. updates) are made to the system and then settings (i.e. for Keyboard and Touchpad) become manipulated, technically.  So, reset both on your Mac, using an external keyboard. You might need this adapter: USB-C to USB A Adapter - Apple