You'll probably have to present an ID confirming you're 18 especially since you're foreign to their country. You might also need to cover your bought eroge on the way back to your country so you won't get blocked in the airport or something.
I think it depends on what kind of eroge you are buying. JAST's prices are between $20-30 but that might not be the case with stores in Akiba. Have you tried looking for japanese stores online? For example, getchu, dlsite, sofmap, toranoana, akibaoo, mandarake etc.