const checkbox = document.getElementById("shipProtect");
  const priceDisplay = document.getElementById("price");
  const basePrice = 149.94;
  const protectionCost = 4.99;
  checkbox.addEventListener("change", () => {
    let price = basePrice - (checkbox.checked ? 0 : protectionCost);
    priceDisplay.textContent = `Only USD $${price.toFixed(2)}`;
  });

<input type="checkbox" id="shipProtect" checked>
<p id="price">Only USD $149.94</p>