I read your input regarding Earth Day.Perhaps there is a way to create "value" to each card so that it goes beyond being a throw-away item.I also sell Greeting Cards locally and forsee the moment when I will start signing each on the back.Time itself then may help the card to accrue in value or worth.This is in regards to selling locally in a place where your name is known.What a shame to see these Beautiful Greeting Cards being tossed into a landfill in the end.Re-useable by all means!!!And excellent idea to insert tag-line.Re-cycle has many paradigm shifts.Bravo

Does seem a bit counter productive...
 
Clever idea of including a message to reuse the card, like "psst, pass it on".  This would be particularly appropriate with a message like "everyday should be earth day" so they're inclined to share the card outside of the holiday.  Maybe the card includes some interesting facts about the earth/environment or the history of Earth Day.
 
In addition to people wanting to keep or frame your lovely card art, perhaps you could come up with some other type of "keepsake" like a bookmark.  I personally use all kinds of odd items as bookmarks.  The front of the card could be designed so the recipient is instructed to cut it into strips (maybe 3) and have 3 bookmarks that ideally would carry a message and bookmark appropriate design.  
 
Just some thoughts  

I agree it's tacky to send e-cards for a variety of occasions-mothers day would certainly rank up there as one of them (it's almost like saying oops, I forgot to get a card or worse, I was too lazy or cheap to get a card).  I'm sure there are certain exceptions to this though, as there are to everything.  Sympathy cards would also rank up there as poor taste as an Ecard, at least IMO.
 
That said, I still think it's odd to send a card specifically to celebrate a day set aside to promote protecting the enviroment.  I don't think it's bad to create and send paper cards in general, just that it's not quite appropriate for Earth Day.  UNLESS there is more to it that just a card.  That was where I was going with this.  Trying to find unique ways to encourage re-use and recycling of the card (whether it's just tossing it in the recycle bin, framing it, passing it on, creating a way to make it a card and a bookmark, etc).  Maybe even something that points out it is a card from a POD, and why that helps the enviroment.  That could even be a push to people to buying from PODs, once they understand the concept and benefits.
 
I thought maybe as Artists, we could get together as a community and brainstorm unique ways of presenting and creating these cards that would not only make them more marketable (I mean, what would you think if you got a paper card telling you to celebrate Earth Day) but also highlight the creativity that is here and maybe find a unique niche that could lead to more sales.  I can image more people sending them if they saw a secondary use or unique message that really promoted the purpose of the day.