That link you sent says that they can charge more if you bring your own casket. One of the options explicitly offered there is for the funeral home to inform potential customers:
"Please note that a fee of (specify dollar amount) for the use of our basic services and overhead is included in the price of our caskets. This same fee shall be added to the total cost of your funeral arrangements if you provide the casket. Our services include (specify)."
It doesn't sound like you can negotiate a price including a casket, but later say you'll make your own casket, saving the bulk of the fee.
"Please note that a fee of (specify dollar amount) for the use of our basic services and overhead is included in the price of our caskets. This same fee shall be added to the total cost of your funeral arrangements if you provide the casket. Our services include (specify)."
It doesn't sound like you can negotiate a price including a casket, but later say you'll make your own casket, saving the bulk of the fee.