Hi Jan,
 
No flaming, but just to explain my own approach.  I do just as you say,
listing the commonest shells at $1.00, because it is hardly worth my while to
import, clean, catalog, pack and ship a shell for $.25.  However, when folks
order shells like Donax variabilis, Littorina littorea, Nassarius obsoletus,
etc. I invariably send out two or three specimens for the $1.00 received.  I
list Gemma gemma at $1.00, but anyone who orders it gets a half dozen or so.
Maybe you are right, and it would be better to simply list them as 3 for
$1.00, I'm not sure.  Would like to hear what other folks think on this.
 
Regards,
Paul Monfils