Good spot.

This does look like it's a bug as we seem to have forgotten to update this post 1.4.0 when we introduced the TransactionAmount property.

Given you are overriding the default ZaroValuePaymentProvider you can probably override the ProcessCallback method with the following implementation as a workaround:

public override CallbackResult ProcessCallback(OrderReadOnly order, HttpRequestBase request, ZeroValuePaymentProviderSettings settings)
{
    return new CallbackResult
    {
        TransactionInfo = new TransactionInfo
        {
            AmountAuthorized = order.TransactionAmount.Value,
            TransactionFee = 0m,
            TransactionId = Guid.NewGuid().ToString("N"),
            PaymentStatus = PaymentStatus.Captured
        }
    };
}

I'll report this on the issue tracker though and get an update pushed out soon.

Thanks for reporting this 👍

Matt

UPDATE: Issue has been raised here https://github.com/vendrhub/vendr/issues/281