In order to prepare your travel to Madrid, you might need a visa. Please check the requirements on this website : http://www.exteriores.gob.es/Portal/en/ServiciosAlCiudadano/InformacionParaExtranjeros/Paginas/Inicio.aspx
Would you need an invitation letter to support your visa application, please ask for it to the conference organizers well in advance. And check with your embassy what information is needed, as this information is changing from country to country.
Electronic copy and printed copy sent by normal mail will be covered by the organisation. If you need a fast courrier, it will be at your charge.
The organizers won't intervene with embassies nor vouch for a participant, in any case.
Invitation letters will only be sent to accepted authors, conference registrants in order of payment or to participants supported by their CIRED national/liaison committee.