Oh right, you're supposed to stick to CDs with doujincore, but if you really want something else and are ready to pay extra for over-sized shipping, then try and get in touch with them. That said, there is an other option which I've just recently uncovered: Japan Shopping Assistance Service are also going to C78. Their prices are about the same as doujincore's but they don't have an item limit or type restriction. However, I have NOT USED THEM in the past, so in no way is this website endorsing JSAS. You're on your own, but feel free to share your experience if you do end up using them.
UPDATE: doujincore will only buy from the "east"/doujin wing, so forget about that MOSAIC.WAV single or the new とろ美/あゆ/ARM mini-album (OMFG). However I have contacted with JSAS and may end up giving them a try just for these two items.