DVLA fines, magistrates fines - ENGLAND

I completely forgot to pay my car tax (£40) for about a year, and got caught last year.

I accepted the first fine of £100 from an outside company (not the dvla) I guessed they were debt collecting on behalf of the dvla. They then phoned again saying I needed to pay again, so I payed the £100 for a second time. They confirmed that this was the final payment.

I then received a letter to plead not guilty or guilty, I pled guilty. Now I have received a letter saying I must pay £500 for court costs. Probably spent around £800 just for a silly mistake, seems a bit harsh. As I usually receive a reminder for my tax but did not receive it.

Is this the normal process? Excessive fine?