Soon as you start to sell shooting you can expect trouble. I guess they shot 3 birds between them (you can't really count non-gamebirds) and figured £20 was a fair price, certainly fairer than £40 a bird...unless they were stratospheric. They had the opportunity too cheat on payment and took it.... nothing you can do about it other than be less trusting in future. Syndicates can often work, selling days is full of pitfalls unless you can guarantee the bag or a 3 times shot count; non game species are either not shot or not charged for...you were naive Sir.