There are two separate issues at work here.
Firstly, the correct cost of postage for these ponies was not £2.85. As I think Sebby or Ringlets said above, it's a post office goof. That's a post office label, it couldn't have been done by the seller at home. Someone at a post office had to print it and stick it on the package when she took it in.
And as everyone from the UK has said, that's not an international mailing label. 2C means Second Class. That's a form of postage only available in the UK. So someone at her post office made an error on charging her. The only thing I don't understand is why you haven't been slapped with an inadequate postage notice, asking you to pay the difference. Unless, of course, that doesn't exist in the Netherlands, in which case, good.
The amount the seller charged you before she posted it was more correct with what it should have cost, if it had had tracking. It doesn't look like she went with tracking or a box, which means it probably wasn't as expensive as £15. But it should have been more than £2.85.
The thing is, although refunding you the shipping difference would be the morally right thing to do, because this looks like an error of the postal service, Ebay can't penalise her for overcharging you. If Ebay check the rates, they'll find that the rate you paid is closer to the real rate than the rate you received. There is also the possibility she did pay the full amount for your item to be posted, but the labels got muddled up. (A receipt would prove that). In which case, she'd be reluctant to refund as she'd be out of pocket. And ultimately she might not be clear herself what the charge was.
The fact there was a missing pony is a different issue. You are owed your pony or a refund for the full cost of pony and her shipping, I would imagine, if she doesn't arrive. The fact the seller has sent her separately may be true if she originally intended to ship like that. The pony may have already been packaged and that's why. But given the mess up over the mailing label, it's possible something else also got messed up here at the post office and that's why you haven't had the packages together. And, if Ringlets has been sent correctly, she will have the correct kind of postage cost on her envelope.
I suspect that what she should have paid for your package plus the postage for Ringlets on her own should add up to around £15. Providing Ringlets arrives.
I am still bemused at how that package got to Holland with the wrong label on it, but I guess you should just take the fact it got to you as a bonus...
It might be that she forgot Ringlets and has now sent her in a separate envelope. I guess wait and see, and if she doesn't arrive, file a claim with ebay?
It sounds as though she has a ton of sales going on and is a little bit all over the place with dealing with them.