Multiple varieties of lilies are extremely toxic to cats and can quickly result in illness and death if any part of the flower is consumed - including the water the flowers are in. 

Many cat parents are not aware that lilies are so lethal.

Recently, my two cats had to be hospitalized after chewing on lily petals from one of several flower arrangements I'd received after my father died. Fortunately, I was able to rush them to a veterinary hospital where they were treated successfully over the course of two days. Had I not seen that the petals had been chewed the same day, it is likely my cats' kidneys would have shut down and I would have had to have them euthanized. 

Because these flowers are so lethal to cats, and they are common in flower arrangements, sellers of lilies should be responsible for sharing information regarding the risks to cats and other pets (they can also make dogs very sick, but may not be as deadly as they are to cats).

Types of lilies that are toxic according to the ASPCA Animal Poison Control Center (https://www.aspca.org/news/which-lilies-are-toxic-pets): 

Acute kidney trauma/death: Easter Lily, Japanese Lily, Oriental Lily hybrids, Stargazer Lily, Casablanca Lily, Tiger Lily and Day Lily

Cause illness, but not necessarily fatal: Peruvian Lily, Lily of the Valley, Calla/Trumpet Lily, Peace Lily, Flame Lily

Simple and effective ways floral sellers can inform their customers:

Brick and morter flower sellers (florists, grocery stores, nurseries and garden centers, etc. ): Put a small sticker or note on the container or with the card in an arrangement warning the recipient that, "Highly toxic flowers are present in this arrangement, keep away from pets," or similar wording. 

Online floral services: Have a disclaimer check box on the site noting the toxicity to pets when lilies or other toxic florals are present, as well as a note with the arrangement. This would alert both the sender and the recipient.
Or: a disclaimer regarding the presence of flowers toxic to cats and other pets on the listing of each arrangement option that includes lilies so that the sender can elect to chose an arrangement without lilies.

Educating pet owners of lily toxicity in a consistent fashion would not be burdensome to the floral industry and it would save the lives of deeply loved pets.