It is common that when someone dies to send flowers and a card. Often sympathy cards containing bereavement poems of mourning for them. But if you choose a card that does not have and you want to write something personal to the card, then you can consider copying a poem of mourning.
It's easy to find a good poem of mourning. You can look on the internet , there are even programs available to help you write your own poem . Poetry has been used for centuries to express how they feel . There are bereavement poems about love , of course , but there are many bereavement poems about loss and grief. Losing a loved one is one of the hardest things I ever live and anyway you can release some of this loss is a good thing.