I'm not opposed to giving the devs some money for a game I enjoy, but I am not going to feed this game quarters like it's an arcade machine. Give me the option to "buy" the game, and I'll give you $5 or something.
If you haven't played Hashi before, please do two things. Read the rules carefully and start with Hashi Light volume 1. There is only one correct answer for each puzzle. If you have connected all bridges but aren't winning, you have two different "continents" (?) that aren't connected to each other. Click and hold on one of the islands, and you will see that only part of the board becomes highlighted.