Tampons are not available for purchase due to the fact that the collars administer hormones to prevent pregnancies and menstruation, as stated in the FAQ:

What are the collar's functions?
The collar functions as a GPS tracking device, a way of administering sedatives and also a way of slowly pumping your bloodstream with a deadly poison that will kill you if you do not eat a candy every three days. Characters will also be administered birth control through their collars and thus will not menstruate or get pregnant.

However, the other items have been paid for from Hina's account!