To my fellow glider’s can you give me some idea how to do this kind of project.
I’m thinking, of like a billing with monthly due date, and when the due date comes it will automatically pop up on their phone and remind them about the due date
Since iOS do not support native notifications for PWAs, I think your best bet is to use Integromat to check your Google Sheet every day and send an email to the billing owner.
It goes like this:
1/Construct a billing Sheet with columns for owner’s email and due date.
2/Add an Integromat scenario that runs 7AM every day to check that specific Sheet for days that are the same as today. Make sure it matches the date/time format.
3/Send email(s) to the owners about the due bills.
What @ThinhDinh said you can do also via Googlescript (if you are using googlesheet), without using any further service. Both options are pretty the same, but if you think you will reach an high volume of emails in a month, then via googlescript you can do this for free with Integromat or similar after a while you should pay.