I ordered a book through 
beNippon that worked out pretty well.  They take PayPal, and can ship either traceable (expensive) or slower and non-traceable (not as expensive, but not cheap).  I chose the slow method and the book arrived in 2 weeks in the USA.  The web pages are in English, which helped on this first-time order.  email support was good too.
The book I ordered cost 890円 and shipping was an additional 480円 to ship 500g, plus a "payment method fee" of 4.5%, 66円.  Total came to 1536円.  It came with rather a lot of bubble wrap for a small paperback.
amazon.jp lists the same book for 780円 plus shipping.  They say free shipping on orders over 1500円 but I doubt that applies to international orders.
The book was 夏子の酒 ("Natsuko no sake") volume 1.  As there are twelve volumes in this manga series, I need to decide eventually the best way to order the rest of them.