You'll have to fully investigate and see if that service procedure is secure. I don't know if they have a secure method of uploading files and sending you the results. You should question them. Their site says it's delivered by email. Well, regular email is not a secure method for sensitive files. So unless they can explain how they secure a sensitive tax document for an email delivery, I'd be concerned about that.
It would also be good to know how well they secure the info once they receive it and store it on their servers. Their phone number is listed in their FAQs. You may wish to talk to them to explain all their security mechanisms, but in the end you may be at their mercy.
