I would only write for people who gave something tangible and would use a generic "thank you for your support during this difficult time for our family" for those that sent money assuming the money is used for gas/food/funeral expenses. if you're rolling the money into a charity you can say "thank you for your gift, we're donating to XXX charity in honour of [relative]."
I also think there should be no sense of obligation to write notes after a loved one dies. you've got plenty of other things to deal with now. I do know some people find it therapeutic to write them though.