The shops are awash with cards covered with pagan imagery, on their own and together. Evergreens, whether firs, yew, holly, ivy or mistletoe are all Pagan in origin. Not to mention the Robins on every other card you see.
Plenty of cards just have 'Seasons Greetings' inside, again this is becoming more common than Happy Christmas. You could always print some labels saying 'Seasons Greetings' to stick over the Happy Christmas message in any cards that says it.
This issue strikes me as being a non-problem