Clams direct also has an email notification thing where you will get emailed the day before Barry posts new clams on his website. You could email them and ask to be put on the email list. From previous experience, the nicest clams often sell within a couple hours from when he posts them, so you need to be quick with the credit card!
As for his shipping, last time I ordered clams, the rate was the same for up to three clams (like $30-40 total for overnight). I am not sure how well a group buy would work just because, again, the nicest stuff gets sold in minutes and it is all WYSIWYG. But, you contact Barry and see if he'd do something special for MARS, he might just do it.
Good luck!