The reason it costs gold to donate and trade max level cards

I'm seeing a lot of players complaining about one feature of this update the most, and that's the fact that you have to pay gold to donate or trade away a card that's max level. At first it didn't make sense to me either; aren't we supposed to earn gold from donating?

And you still are. It's just hard to tell.

One feature of the Star Points that they didn't clarify very well is that you can no longer hold extra cards above max level. When the update went live, all extra cards were converted to gold. Whenever you earn a card that is already max, it gets converted directly into gold. If you're level 13, it gets converted into Star Points instead.

Because of this change, players could no longer donate and trade cards that were max level, because they couldn't hold spare cards. So the Clash Royale team came up with a solution: If you want to donate or trade a card that you have maxed, you can buy it as long as it is immediately used. That's what the gold cost is: Not paying to donate a card, but paying to buy the card and immediately giving it away.

And it's always a net gain, as the team explained in this post. Take a max rare card, for example. When you earn a rare card that is already level 13, it gets converted into 50 gold. If you want to donate it, you pay 50 gold to buy it back, then you get 50 gold from donating it. Overall, you still end up with +50 gold. (Or, you could look at it as buying any card, then donating it to earn the same amount of gold; basically, any level 13 donation is free.)

TL;DR: If you're upset that Supercell took your extra cards that were for trading, or that they make you pay to donate, they're actually not doing either. All the extra cards were converted into gold, and you can buy them back any time to donate or trade.