So keep using Excel. Don't use Google Sheets.

I don't use Google Drive but it's just an online file storage system, isn't it? Can't you store Excel files in it? If not, don't use it. Maybe use Microsoft's OneDrive instead.

I said their behaviour was rational. I didn't say you or anyone else would like it.

If your client's action in choosing Google Drive and using Google Sheets is causing you problems, explain to them why you'll be charging them more.