Two different problems, and each has a straightforward answer.
Nothing is arriving
Check the spam folder first. Every time. It is the answer more often than anything else on this list.
Then suspect WordPress. This is not our plugin being awkward. WordPress is genuinely poor at sending email: many hosts restrict it, and the mail that does escape is often unauthenticated, which means spam filters treat it with suspicion.
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, it costs nothing for the volumes you will be sending, and it fixes email across your entire website rather than just ours.
If you send anything at all from your site, this is worth doing regardless of us.
Check your sender address. 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.
Check the email is switched on. Each one has a Send this email toggle. It may simply be off.
Everything is arriving twice
Not a bug, and there is a simple reason.
Houzez sends its own emails, and so do we. Both systems noticed the payment. Both did their job. Your customer got two.
Houzez’s emails live under Theme Options, then Email Management. Look for Package Activated and Purchase Activated.
Ours live under Mollie Payments, then Email management.
Pick one set and switch the other off.
Which set to keep
Most people keep ours, for three reasons.
They carry the invoice.
They can be designed to match your brand, if you are on Business or Agency.
They cover the whole lifecycle, not just the purchase. Renewals, warnings, failures, expiries and cancellations. Houzez has no equivalent for most of those, because Houzez was not built to run subscriptions.
But it is your website, and there is no wrong answer as long as your customer only receives one.