How does buying a game from Greenmangaming work?

Could you please elaborate on the process of purchasing a game from Greenmangaming? Specifically, what steps do I need to follow from start to finish? Is there a particular payment method that's preferred? Are there any discounts or promotions available? Also, how does the delivery process work? And finally, what happens if there's an issue with my purchase? I'd appreciate a detailed breakdown of the entire process.