You get an email sent to you for every purchase you make in iTunes. Check your email history. you should also have a credit card record if you didn't use an iTunes Gift Card.
Second, Navionics posted an update to the app February 11, 2014. If you have not updated already, you should do so.
If that doesn't work, contact Navionics directly for support with their app: http://navionics.com/en/customers