You pay for the privilege, and the transmission system is open. The Hydro-Québec marketing arm deals with all the niceties involved in selling the power through whatever interconnection is the most viable at the lowest cost to the buyer, so that all works quite well.
The only challenge with it is that we need to take your DC power and change it on the way through the interties, but we have big investments in that at the interties that do that. So yes, that does increase the cost a little bit for Quebec to export its power, but by the same token, Quebec makes a fair amount of money. They've never told me how much they make, but I think Hydro-Québec does very well, thank you very much, selling power to all the rest of us.