As far as difficulty goes I'd rather not charge for that. Seems silly to me.
it isn't silly. you said it yourself that harder bricks take longer to make, and therefore it winds up costing more. it's basically just taking your shady, crappy hours system and solidifying it into a solid sort of deal. you assume a difficult brick takes 3 hours, so you charge for 3 hours instead of 'hoping' it takes 3 hours. the only "silly" thing here is the thought that you'd actually try to make a payment deal with someone before they even know what it costs.
there's not a single legit service that i've seen run like this. even other similar request services i've seen/used had a flat price, or a price negotiated BEFORE work starts.
I don't see how that's related. I'd be charging for the time and resources it took to make the candy, not for how long it takes for me to fiddle around with your coins. :\
here how about I put things into a more reasonable perspective.
this girl offers a similar service, except with sheet music. there's a flat price, regardless of the song difficult or "hours POSSIBLY spent" working on it. you add 10 to the price for songs over 8 minutes. if you want the sheet music fully arranged and playable, it costs more than just having the song done. now imaging if she charged per hour spent working on a song (she even stated herself that they can take a long time to make). prices would be completely unreasonable and she'd just be able to say "oh sorry it took a LONG time i'm going to have to charge you more!!". then would arise the problems of "oh well I doubt you worked THAT hard you're just trying to make us pay more!" and "oh I DID work hard you have to trust me!!".
having flat rates makes sense, or at least have a scale that can be determined before work actually starts. Say someone wanted a pyramid brick. You'd factor in the detail of the brick, collision mesh, grid build-ability, texture complexity, and script extent, then come up with a breakdown and explain why it costs that much and how long it might take. That's what a legit service would do.
You don't just hope it doesn't take long and charge extra if it does.