It's not my opinion that it's a purchase, it's a fact. Your own payment system uses the verbiage "buy coins." Might want to change that to avoid issues.
Not sure how you can argue that it's donating. Again, points to verbiage in your own system.
As for voluntary, seriously? Again, all purchases ever are voluntary. No one's making me go to the Walmart and buy 50-12 packs of toilet paper, I'm doing that on my own. What this is is a free to play game with micro-transactions (now that's an opinion, although mechanically it's exactly the same.)
Was just pointing it out for all of the people saying "well, it's a donation - they don't have to honor it or care if it doesn't work."