You wouldn't want to purchase multiple exp coupons of the same kind at once, that would be a waste.
The only item that you would want multiple of is the Gacha tickets which you can buy many at once.
We will make a 20 pack for the Miwok artifact though as you do need to buy that many times.

Live now, following Mark's call above: coupons stay one at a time, and the Enchanted Scroll for Toh Relicseeker's Miwok seals now also comes as a 20-pack on the Cash Shop's EVENT tab (Event page), 20 VX - the same price per scroll. Enchanted Scrolls now stack in one slot too, so the pack takes a single Cash slot.