charge off account with interest

california


few years(98) ago i got auto insurance and it was through some finance company(i didn't know what that was)
paid few month but i cancelled and they sent me a bill to pay the interest that they financed...first i ignor then they sent to collection agent
finance charge(interest to pay) was about $300...i just checked my credit report and it went up to about $500 so i called the collection agent to settle the amount and i asked few questions and they all ignored

asked
if i can make payment (they said they don't take payment)
they only wants me to send in via money gram(wire)
so i asked if they can knock off the wire fee($10) NO
asked why they're charging interest...they said its up to them by law

is it legal to adding interest???


so basically they are asking pay at once or save up and pay later with more interest

is this legal? whats my rights?