The invoice is supposed to be the actual price paid for the horse.
Customs can go to your web site and check prices. Maybe you don't have prices on your site but I just don't think an incorrect invoice is worth the worry.
We have brought horses into Ontario from the U.S. as recently as October 28th, 2010 and the paperwork I mentioned in my earlier post was what we were advised to have.
Customs only seemed to be interested in the vet paperwork and the invoice.
I think they must be talking about the Federal Vet paperwork when they say "Exportation papers"