The server that is used to send emails is the WordPress server. Sometimes for unexplained reasons, things just don’t work properly at one time or another. If you test again and things still aren’t working properly, you do have another option.
Thanks for the quick response. I will try this option if this is more
stable. Also, for the quantity of tickets. Is there anyway you can set 1
as default? If customer forgot to select number if ticket, it will prompt
them to select one, then the button go back to the empty shop not previous
page. I noticed it happens on your demo site as well.