Have you checked into Square Trade?

https://www.amazon.com/SquareTrade-Consumer-Electronics-Protection-1000-1249-99/dp/B001N82KS8/ref=sr_1_1?fst=as%3Aoff&qid=1556645891&refinements=p_89%3ASquareTrade&rnid=2528832011&s=pc&sr=1-1

Extended coverage is extremely important for Pixelbooks. If you can't add the coverage now, I would encourage you to consider returning it yourself and then re-purchasing it with additional coverage, or do whatever is needed to end up with proper coverage.

I would also strongly encourage you to return it to Amazon - they are just about the worst vendor for Pixelbooks. Your mom should understand.