This is pedantic but you're thinking of "romanization", the act of transliterating Japanese with the Roman alphabet (romaji). There are different systems of romanization, most notably Hepburn and Kunrei. In Hepburn shi is correct, in Kunrei si is correct.
It sounds like you're saying "si" are not valid characters in the Roman alphabet.
I got Calrose about a month back for just over 3000 yen, which is slightly cheaper than the homegrown stuff. The issue is that a year ago the homegrown stuff was going for 1500 yen.
It sounds like you're saying "si" are not valid characters in the Roman alphabet.