They will exclusively sell a T-shirt ($40) and a Poster ($20)
They will also exclusively sell a Glow in the dark vinyl ($40) and a The Forum sleeve CD ($20)
This is on top of the regular US tour merch:
Joakim Brodén, lead singer of Sabaton, and featured artist on "Oh! MAJINAI" from METAL GALAXY, talks a bit about the process of recording for it.
It appears to have been a very off-hand experience. He explained the process in a talk with Herman Li of Dragonforce.