Javascript/オーダーフォーム

const address = document.getElementById("address");
const s1Num = document.getElementById("s1Num");
s1Num.style.width = "50px";
const s2Num = document.getElementById("s2Num");
s2Num.style.width = "50px";
const s3Span = document.getElementById("s3Span");
const s4Span = document.getElementById("s4Span");
const s5Span = document.getElementById("s5Span");
const sum = document.getElementById("sum");
var postage = 0;
var s1Sum = 0;
var s2Sum = 0;
address.addEventListener("change", function(){
  if(address.value == "北海道"){
  	postage = 1000;
  	s5Span.textContent = postage;
  	sum.innerText = s1Sum + s2Sum + postage;
  } else if(address.value == "本州"){
  	postage = 500;
  	s5Span.textContent = postage;
  	sum.innerText = s1Sum + s2Sum + postage;
  } else if(address.value == "九州"){
  	postage = 800;
  	s5Span.textContent = postage;
  	sum.innerText = s1Sum + s2Sum + postage;
  } else {
  	postage = 0;
  	s5Span.textContent = postage;
  }
});
s1Num.addEventListener("change", function(){
  if(s1Num.value < 0){
  	s1Num.value = 0;
  }
  s3Span.innerText = s1Num.value;
  s1Sum = s1Num.value * 1000;
  sum.innerText = s1Sum + s2Sum + postage;
});
s2Num.addEventListener("change", function(){
  s4Span.innerText = s2Num.value;
  s2Sum = s2Num.value * 1500;
  sum.innerText = s1Sum + s2Sum + postage;
});