有几点需要修正
1.你的item_cost_price是未定义的，在这个例子中，我假设你在某个DOM元素中有这个值，所以我用这个const item_cost_price = Number(document.getElementById("milk-item-cost").innerHTML);从它那里得到值，并将其成本保存为全局值。如果不是这样，那么你可以用其他方式定义价格的值，但在这个例子中，让我们这样做。
1.你有一个addamount()函数来获得特定项目的总价，但你没有一个函数来从总价中减去价值，所以我添加了它，并在<td><button onclick="removeCount('milk-count'); decrementamount('milkcost')">-</button></td>中调用它
1.使用parseFloat而不是parseInt，因为parseInt将舍入数字，当按下按钮时，您将无法获得正确的总额。

const item_cost_price = Number(document.getElementById("milk-item-cost").innerHTML);
/*add count to list*/

function addCount(countId) {
  var count = document.getElementById(countId);
  count.innerHTML = parseInt(count.innerHTML) + 1;
}

/*remove count to list*/

function removeCount(countId) {
  var count = document.getElementById(countId);
  var currentCount = parseFloat(count.innerHTML);
  if (currentCount > 0) {
    count.innerHTML = currentCount - 1;
  }
}

/*add price form menu*/
function addamount(item_cost) {
  var count = document.getElementById(item_cost);
  count.innerHTML = parseFloat(count.innerHTML) + item_cost_price;
}

/*decrement price form menu*/
function decrementamount(item_cost) {
  var count = document.getElementById(item_cost);
  var currentAmount = parseFloat(count.innerHTML);
  if (currentAmount > 0) {
    count.innerHTML = parseFloat(count.innerHTML) - item_cost_price;
  }

}

/*clear count chart*/
function clearCounts() {
  var counts = document.querySelectorAll('td[id$="-count"]');
  counts.forEach(function(count) {
    count.innerHTML = '0';
  });
}

/*sends the total */
function submitCounts() {
  var milkCount = document.getElementById('milk-count').innerHTML;
  var eggsCount = document.getElementById('eggs-count').innerHTML;
  var juiceCount = document.getElementById('juice-count').innerHTML;
  var total = milkCount + juiceCount + eggsCount;
  alert('Milk: ' + milkCount + '\nEggs: ' + eggsCount + '\nJuice: ' + juiceCount + "\nTotal:" + total);
}

<div id="milk-count">0</div>
<div id="milkcost">0</div>
<span>Price for current item</span>
<div id="milk-item-cost">2.84</div>
<td><button onclick="addCount('milk-count');addamount('milkcost'); ">+</button></td>
<td><button onclick="removeCount('milk-count'); decrementamount('milkcost')">-</button></td>