The best form is soluble or slightly insoluble. The total amount of antigen should be send in several 2 ml eppendorf tubes containing 0.250 mg  each of antigen in PBS or other buffer. It is not necessary to send all the antigen at the start of the project;  more can be sent later.
In Portugal: we recommend Post Office service for over night delivery  (Correio Azul) or an express carrier (blue and dry ice packages).
In Europe/USA/Canada or rest of the world: please, use a reliable carrier.