Now that the prototype has proved mostly successful, has it crossed your mind to stop paying it yourself and have people need to get the api code themselves full stop? You could just link to the tutorial you made to get the free code. It might make people a little annoyed but it still remains free and you can expand it properly.
It might help to add the tutorial graphic in your post in the game itself, maybe either as a drop down on the main menu or in the settings tab by where you enter the api code? The hardest part about using the guide was finding where the link was hiding. At the very least it means its very accessible so you get less comments about it.