Right now I handle spam by not having a recurring tweeting function ;) You have to manually send each tweet, however that's not to say someone determined (not to mention, also very bored) couldn't sit there constantly hitting the send button. I'll look in to this.

I'm going to play around with where else to put the price as you're right, it's a bit hidden at the moment.

Currently, the only way to pay is to log in with a Twitter account first and then upgrade from there. I'm thinking about an option to "sign up for premium" where after signing in with Twitter the user's taken to a payment page. Do you think that'd be a good idea?