function validateZIP(field) {
var valid = "0123456789-";
var hyphencount = 0;

if (field.length!=5) {
alert("Please enter your 5 digit US zip code. For APO/FPO, Puerto Rico and International rates, enter 00000.");
return false;
}
for (var i=0; i < field.length; i++) {
temp = "" + field.substring(i, i+1);
if (temp == "-") hyphencount++;
if (valid.indexOf(temp) == "-1") {
alert("Invalid characters in your zip code. Please enter your 5 digit US zip code. For APO/FPO, Puerto Rico and International rates, enter 00000.");
return false;
}
if (hyphencount!=0) {
alert("Invalid characters in your zip code. Please enter your 5 digit US zip code. For APO/FPO, Puerto Rico and International rates, enter 00000.");
return false;
   }
}
return true;
}