Hello there,
I'm from Chile, and I worked in USA under a J1 Visa on 2007.
The question is the following,
I did my refund taxes with TAXBACK.COM, they made all the paper work, and I authorized them to received my check. Reading around the forms it says that is illegal to cash my check by anyone but me.
When I ask to TAXBACK.COM they answer me that I authorized them!!
The forms I filled where: 2848 Power of Attorney - 8822 Change of address - 8821 Tax information authorization. But any of this said nothing about cashing my check.
So,
They received my check because I made the change of address, but I don't know how they did to cash it, is that possible? Is there any authorization to do that?
Thank you very much for your answer.
Chachio
P.D: Is a hard one!!

Yes, if you gave them power of attorney over the check, they may cash it for you. Power of attorney allows them to sign for you.

Even when you sign Form 2848 and authorize them to receive your check, they are not allowed to cash it. Only way possible is if you endorse on the back of the check.
The tax preparer may be holding your check and may want to to pay their fees before they hand over the check to you.
If they cashed it call IRS and inform them.