"I expect to know who's generating that random number" - this is generated by Ruby, as described in /fair, using the code
I don't understand who's generating the seed. If I do it locally, and hand it over to you, with source code transparent, can't I cheat the game? Analogously, if I don't do it locally, and you hand me over the seed, can't you cheat me?
Are you sure you followed steps 1 and 2 in order? If you make a request (step 2) before paying the invoice (step 1) it won't execute...
I didn't. I just tried to pay the invoice as a first step. Just tried again, but wasn't possible neither.