My son was cremated and his casket cremated with him. I did not want the casket just him. The funeral director said that is the way it is done. I think they just wanted to make money on the casket. I have been told by others that you can rent a casket and then they are taken out of the casket and cremated. Of course the funeral parlor told me they can not be responsible for the condition that the casket arrives in. Another gimmick no doubt for the funeral parlor to soak you at a vulnerable time.
I noticed our local funeral home had plain, fairly inexpensive coffins that were listed as suitable for cremation - although I have read about casket rentals I haven't heard of anyone who offers this.
And, if it’s any consolation, both had a prepayment plan with the funeral home. My mom paid into it for decades. I estimated she paid over $10,000. When she passed two years ago, it still cost us $7,000 for the cremation, burial and headstone. I will always believe we were royally ripped off.
And the casket cremation does sound like an extra charge that could have been avoided perhaps, if they had let you know. Sorry you're dealing with this on top of everything else.
Thinking of you.