Add tiers in the Group deal tab, one per line as group size = price, for example 15 = 419 and 20 = 399. The deal price unlocks at the target; each tier lowers it further. The next buyer pays the price of the group including them.
Best price for everyone. When an unlocked deal ends, the final price is worked out from the final group size. Buyers who paid more get the difference refunded automatically and are emailed. Refunds the payment method cannot make are listed in the reports.
