So you paid a mutually agreed price for a funeral. If you didn’t want their services you could have gone elsewhere.
There is no agreement involved when you're told your mother wouldn't be buried with what you perceive as dignity, and you're manipulated while in a state of shock and grief to think that the voluntary payment you're making is mandatory.
Would she have felt it was her children's duty to pay more than necessary to be buried a certain way? I'm not asking this rhetorically. I really am curious. We all want a dignified funeral for our loved ones, but did grandma make her wishes known?