USD and GBP are possible too, but must be enabled for the shop first by MK Admin. It is a premium feature.
The transaction amount is converted into EUR in the moment of its creation.
Such transaction will have additional attributes 'original_currency' and 'original_amount'
See more on EUR and USD payments on multicurrency feature

Specifying a country will filter into the response message only in the payment methods relevant to this country.
If omitted the system will default to Estonia (EE).

Each shop has its set of urls managable in Mercant Portal under the API settings
You can optionally over-ride these urls when creating the transaction
It might be handy at some e-commerce platforms where otherwise it is difficult determine previous context (i.e. user language)
Or situation when multiple develpers are working on integration of one shop on their local work stations.

Be careful to provide correct urls, with protocol (i.e. https://myshop.com/returnhandler/). We reccommend to use POST method.
If already, then you have to specify full set - you can not pass just one of the 3 urls (though they can be equal)