Unfortunately, even with a discount of 10% on a group buy of 20+ units, the postage to send them out to everyone would make doing Autan a no go'er, would make more sense to buy the bottle from the site as individuals, or getting them from a local shop. Not asked the Smidge people yet on their stance on group buys.
Yes, carriage is always going to be a problem for smaller items. It might work if there was an upcoming meet and everybody in the group was going. If I'd had a sack of insect repellent bottles at North Wood last weekend they'd have sold like hot cakes.