From the
What do you pay for new Japanese MPs? thread:
You need to have a Japanese address, they don't ship overseas. They're preorder prices are quite good and they ship domestically for free. So it's not an available option for people who don't have access to a Japanese address. But Amiami and HLJ ship internationally, and they charge either full RRP and nowadays below RRP if you preorder; I've never seen them charge above RRP for Transformers. I only started using Amazon JP last year, before then I used to use eHobby to occasionally order some Japanese TFs that were harder to find in stores, but eHobby's gotten pricier now, so I only use them for exclusive toys (e.g. United Magnificus, who's also really expensive); otherwise I've now switched to Amazon JP for a lot of my Japanese TFs now. eHobby also only ships to Japanese addresses.
For those without access to a Japanese address, I'd happily recommend HLJ.com. I've used them plenty of times for 10 years now and I've never had a problem w/ their service. I've used other online stores like Robot Kingdom, Amiami and BBTS, but only once or twice each so I think my experience with them is too limited to really give a good appraisal of their services.