Building a custom payment plugin for Webform [on hold]


I am looking for the best approach to building a custom webform 8 payment option.
It would need to redirect to the payment service URL once form is submitted, but not send emails or show confirmation until the correct URL comes back from Payment service. At which point I would update the submission with returned details, either payment excepted or payment failed then send an email.
Just not sure where to tap into Webform. I have been looking at a custom handler using submitForm but that still sends emails regardless.
Any hepl appreciated.

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