This is different than my experience ... since i've owned Navigon w/ traffic on 3G and transfered to 3GS and reloaded Navigon a couple of times on each device ... i've hit the Purchase Traffic a couple of times, and it reactivates on the device w/o charging me ... and most recently, i hit the Restore Purchase and it activated traffic again ...
here's what i would suggest ... delete app, do a master reset (home button and power button for 10 sec), then re-sync app, then do a master reset again (just gives you the cleanest start to using app), then open navigon and check traffic ... either purchase again, or restore ... since there is a record of your purchase on iTunes (which you can double check purchase history), you shouldn't get recharged, and if you did, you'll be able to handle w/ Apple support.
again, i have not heard others w/ this issue - you should be able to get is active again ... good luck