See also: Order attribution in Ometria
Ometria still receives the order through your ecommerce data even if your payment page redirects at the point of checkout (e.g. to Paypal, Sage, Gumroad etc.)
However, it is possible that this order will be incorrectly attributed.
For example, if the contact is on your site, clicks ‘pay via PayPal’, then goes to paypal.com and comes back to your site, it is possible that the visit is attributed to paypal.com based on visit based attribution, as that's the last source of a visit.
This issue is more likely to occur in Google Analytics than in Ometria, but it can happen.