Is this ethical?

I just recently found out about a derogatory reporting on my credit. It's from a vet bill from a very traumatic experience. The vet said she would bill me cause of everything that was going on. Well apparently they had my old address and it ended up going to collections a couple months ago. The CA is not willing to work me at ALL even considering I just found out about it. Fine I have no problem paying it all but then they added on over of $100 worth of interest and said if I don't pay the interest they will not "delete" it from my credit history. If I just pay the original amount it'll stay on my credit report. I'm pretty sure the original bill didn't have any interest agreement....they're charging me 10% per day! I'm pretty sure what they're saying is a FDCPA violation "They cannot ask you to pay interest, fees, or expenses that are not allowed by law - The collector can't add on any extra fees that your original credit or loan agreement doesn't allow. [15 USC 1692f] § 808(1)"

Any input, please? I'm in California.