请老猫看:

My wife and I bought a 05 Toyota and registered with the DMV of State of R.I. in August 2005. Subsequently, we moved to Mass and then regisgtered our car with Mass and changed our drivers\' license as well, while still keeping the RI plate. In Septmebr 07, after we recieved a letter from R.I. law office, which collects the exercise tax on behalf of RI state, we realized that we needed to return the plate to state of RI. So we paid the state / law firm the due (about $500) and returned the plate in October 07. we thought that it should be over upon the receipt of the car plate by the DMV.
However, to my surprise, we received a notice from the same law firm again this afternoon, demanding a $610 due for RI exercise tax again (with a due date as 7/1/07).
We would like to know, if possible,
1) since we returned the plates (although we did not physically go back to RI for the cancellation of the registration) and properly register our car in Mass in 06, are we still liable for such payments?
2) if we do not bear that liablity, how can we fight it?
3) it is such a mistake that we could not find our return recipt/ mail conformation (which happened a year ago..)
3) we have not yet get a chance to talk to the law office but we are wondering if you or any other experts could kindly shed some lights on this issues.

Thanks for your time and any feed back is very much appreciated!