@Pantelis - typically this is done in the PaymentGatewayMethodBase which internally uses a private method to assert the invoice status:

I think in your case, the method can be simplified since you know it is an authorize or capture by your result during execution, but the entire method is

 /// <summary>
    /// Validates the invoice status is correct after a payment request
    /// </summary>
    /// <param name="invoice">
    /// The invoice.
    /// </param>
    private void AssertInvoiceStatus(IInvoice invoice)
    {
        var appliedPayments = _gatewayProviderService.GetAppliedPaymentsByInvoiceKey(invoice.Key).ToArray();

        var appliedTotal = 
            appliedPayments.Where(x => x.TransactionType == AppliedPaymentType.Debit).Sum(x => x.Amount) - 
            appliedPayments.Where(x => x.TransactionType == AppliedPaymentType.Credit).Sum(x => x.Amount);

        var statuses = GatewayProviderService.GetAllInvoiceStatuses().ToArray();

        if (invoice.Total > appliedTotal && invoice.InvoiceStatusKey != Core.Constants.DefaultKeys.InvoiceStatus.Partial)
            invoice.InvoiceStatus = statuses.First(x => x.Key == Core.Constants.DefaultKeys.InvoiceStatus.Partial);
        if (appliedTotal == 0 && invoice.InvoiceStatusKey != Core.Constants.DefaultKeys.InvoiceStatus.Unpaid)
            invoice.InvoiceStatus = statuses.First(x => x.Key == Core.Constants.DefaultKeys.InvoiceStatus.Unpaid);
        if (invoice.Total <= appliedTotal && invoice.InvoiceStatusKey != Core.Constants.DefaultKeys.InvoiceStatus.Paid)
            invoice.InvoiceStatus = statuses.First(x => x.Key == Core.Constants.DefaultKeys.InvoiceStatus.Paid);

        if(invoice.IsDirty()) GatewayProviderService.Save(invoice);

    }        

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