To me, the only 'ethical' things you can sell in as an add on to a game are levels, weapons, badguys and a level editor. You shouldn't sell a main point of it, but an improvement of one that's expected.
I personally would never get out a credit card for a flash game upgrade, but that's because I'm incredibly autistic when it comes to credit card details, but I would definatly use my phone to spend a quid or two on something genuinely worth it, as it's less direct and no credit cards would be needed. If Portal: The Flash Version released an extra 25 levels for a pound, I would get out my phone to buy it on the spot, but this would only be true for a handful of awesome games.
What I do think should be added more on flash games are the 'buy me a coffee' type things that are on blogs, where the user just donates straight to the designer a pound or two. Not sure how effective they are, but they are probably worth adding.
ALSO: I'm making a full API game that's fast paced that I want to release as a proper game. Do you think it's a good idea to add sound, even though it will stop it being a 'true' API game (as there would be something in the library)? I'll kinda feel dirty if I do. I want the library to be clean. I considered hosting them on a server and loading them from there but Newgrounds threw loads of sandbox errors when I last made a game when I tried to do that.
I know I used 'API' in the wrong sense but for some reason everyone uses it in this way when talking about flash, and you know what I'm talking about.