Install and set pos id, merchant id (it seems to be the same as pos id), crc id, api key in the plugin settings.

The default country code is "PL". You may set any other value in the order form field named "country". The default language code is "en". You may set any other value in the order form field named "lang".

Address, zip, city, phone fields will be set if there are according fields in the order form.