If I use share-it, it says I must pay 9 Euro for Soldat(You can't change the type of currency), which is 10 dollars and 78 cents. Should I pay 9 dollars or 10.78$?

9 Euro. Share-It is a bit more expensive (by 1.78$), so use Digitalcandle if you don't have to.

I can only use Share-it.
Can I send them 10.78$ in US dollars, or must I somehow convert it to Euros.

You want to send by mail? The best thing to do is to contact Share-It, they'll explain everything.