<form name="shoping" method="get" onSubmit="sracunaj(this)">
              // ....
              <input type="checkbox" name="buy"  style="width:30px; border-width:1px" align="right"/>
              <input name="kolicina" type="text" style="width:30px; border-width:1px; text-align:right" value="1" maxlength="3" />
              // ....
              <input type="checkbox" name="buy" style="width:30px; border-width:1px" align="right"/>
              <input name="kolicina" type="text" style="width:30px; border-width:1px; text-align:right" value="1" maxlength="3" />
              // ....
      <p align="center">Ukupna cena: <input name="ukupno" type="text" value="0" style="width:80px; border-width:1px; text-align:right"/> &nbsp;din.</p>
        <p align="center"><input type="submit" name="potvrdi" value="Potvrdi naručivanje" /> <input type="reset" name="reset" value="Resetuj polja" /></p>
      </form>

<script type="text/javascript">
var cena=0
var curCena=0
var tempStr
var sifre=[ //sifre za prizvode,redom od prvog do zadnjeg
'0001',
'0002',
'0003'
]

function getcena(i){
switch(sifre[i]){
case '0001':
curCena=100; break;
case '0002':
curCena=150; break;
case '0003':
curCena=200; break;
}
}

function sracunaj(curobj){
for (i=0; i< document.shoping.buy.length; i++){
if (document.shoping.buy[i].checked) {
getcena(i)
tempStr=document.shoping.kolicina[i].value
cena = cena + (curCena * (+tempStr))
}
}
document.shoping.ukupno.value=cena
}
</script>

<form name="shoping" method="get" onSubmit="sracunaj(this)">