If you have any questions about forwarding services, I use them quite often. Most should keep the price of the item you're buying the same, however, and just tack on a service fee. They're usually pretty clear about what's what in the breakdowns of their invoices.
I use Tenso the most just because it also can serve as your own Japanese address if you need it (the address they give you is a bit personalized (it's a warehouse, but your Tenso address includes your user ID in the form of numbers) so anything you buy that gets sent to it, they immediately know it's yours) -- the only downside is that you need to provide ID proof, and it takes some time to get confirmed. I just used my driver's license and they verified me the next day, but I've heard different experiences. Their service fee varies and they offer extra things on top of it, but in my case it's usually between 300 and 600 yen.
Outside of that, I've also used Zenmarket which is okay, though I've heard a few complaints. I haven't had much issue with them personally, except I'm not a fan of the wallet thing where you have to put in money in order to spend any of it, and any money you don't spend that you want refunded has to go through a process. Usually I just put in the exact amount I need and it's not a problem, but if you want to bid on an auction on Yahoo and you lose said auction... putting the money back is annoying orz. Their service fee is 300 yen per item. Not a problem if you're buying one thing (in this case, a Vocaloid), but it really adds up if you're buying multiple things.
I've also used Big in Japan, which I think is good but also gives me anxiety orz. There's only one physical address to send your item to, and they know it's yours by you buying a ticket in advance of them receiving your item and describing in detail what the item(s) is/are. I've never had any issue or problem in doing this, they've always found my stuff just fine, but still... it's a bit nervewracking, aha. That's just them as a forwarding service though; they also have a proxy service where they actually buy the item for you, but I've never used it myself. I think you have to email them all the details, but there should be more information on their site. Their service fee (aka price of the ticket) is a flat 500 yen.
There are a few more forwarding services I've tried, but these are the big three. If you do go with one of them and have any questions, let me know! I'm pretty familiar with them all.