Two different problems, so find yours below.
Nothing is arriving at all
Check the spam folder first. Genuinely. Do this before anything else.
Then suspect WordPress itself. This is not our plugin being difficult. WordPress is notoriously bad at sending email: many hosts do not allow it, and the mail that does get out often lands in spam because it is not properly authenticated.
The permanent fix is an SMTP plugin, which sends your email through a real mail service instead of through your web server. It takes ten minutes to set up and it fixes email across your whole website, not just ours.
If you send anything from your website at all, this is worth doing regardless.
Check the emails are switched on. In the plugin, go to Email management and make sure the ones you expect are enabled.
Check your sender address. Also under Email management. If your from-address is at a different domain than your website, some mail servers will reject it outright. Use an address at your own domain.
Everything is arriving twice
This is not a bug either, and it has a simple cause.
Houzez sends its own emails, and so do we. Both systems noticed the payment, and both did their job.
Houzez’s emails live under Theme Options, Email Management. Look for Package Activated and Purchase Activated.
Ours live under Mollie Payments, Email management.
Pick one set and switch the other off.
Most people keep ours, because they include the invoice, they can be styled to match your brand, and they cover the whole payment lifecycle including renewals, reminders and cancellations.
But it is your choice, and there is no wrong answer as long as your customer only gets one.