The Hoverpen is compatible with any medium ballpoint cartridge. We recommend Cross brand or similar if you're looking for a non-Shanpo replacement. Please take a look at the dimensions of the cartridge on the campaign page for the ideal cartridge size.
Simply select a pledge level, add the value of the add-ons to your pledge, and then click Continue. After the campaign, you'll receive a link to a pledge manager survey where you'll be able to confirm your add-ons and finalize your pledge.
Titanium is very strong - it has one of the highest strength to weight ratios of any usable metals. Titanium is heavier than aluminum, and has a very high quality feel. Titanium is used in rockets and other spacecrafts - so having a titanium Hoverpen is like having a tiny spaceship on your tabletop :)