Error on page 6 middle panel left bubble it reads "I don't know what do." shouldn't it be "I don't know what to do." when she's talking? Over all a great read.
I'm personally guessing from the dialogue on Pages 15 and 16 that Kazuhiro at least is employed somewhere, ergo the money to order the (ahem) "merchandise" is present ... otherwise, some parent is likely to exclaim "WTF?" when their credit card or debit card bill comes in, containing details of this transaction, especially the "extra item".
That aside, this was cute enough to get the job done.
Well, the classic joke is that the girl with large breasts complains about how much of a pain they are (e.g. stiff/sore shoulders), and the girls with small breasts want to kill her.
That being said, while I love big boobs, there's got to be a limit past which having them be any bigger just makes life harder on the girl without really benefiting the guy (or helping the girl get a guy). And on that basis, her breasts are probably a bit too large, but her boyfriend sure enjoys them, and complaining about them helped her officially nab him. :)
And in reality, most girls don't get breasts anywhere near that large without enhancement. But with hentai, reality doesn't matter so much...
I mean, sucks he had to spend all that money on underwear but he'll probably be happy he did in the long run when it prevents her chest from sagging as fast lol.