Question : What medium should I use to illustrate greeting cards? Is it ok to use different mediums and surfaces, such as acrylic on canvas?

Answer : There aren’t any restrictions on what mediums you use to illustrate your greeting card designs. Whether you illustrate with watercolour on paper or oils on canvas, what will be the deciding factor is how awesome your illustration looks. Just consider how easy it will be to digitize your illustration (whether it be by scanning or photographing) before you delve in. Other than that, the sky’s the limit!
You can find more tips on illustrating for greeting cards here.