Marta,
There are three ways it's done at AKC shows:
1. the photographer sends out a finished 8X10 along with a bill and hopes to get paid. Then spends the next however many weeks until the next show trying to collect. All bills get paid before the next show where that particular photographer is shooting or no other photos to those who haven't paid.

2. the photographer sends out a set of proofs and the exhibitor orders the products (prints, merchandise, etc.) they want. It's prepaid but the exhibitor gets to see what they are paying for.

3. the photographer posts the images on a website the exhibitor orders product based on these web-proofs. Again, it's prepaid but the exhibitor gets to see what they are paying for.

Here in the states, the show committee never pays the photographer to shoot the show (in fact, we pay an "incentive" to the club) and sales are a function of how good a formal shot we got. As for people shooting over our shoulders at AKC shows, I add in my signs (with the award won, date, show logo, etc.) only AFTER the exhibitor has ordered their product(s). It's the only control we have over the owners who think it's OK to shoot their dog over the OP's head/shoulder.

Jim