Struggling with writing Javascript if/else statement.

Be careful about multi-line statement blocks without using braces, such as your if/else statements. I don't know how you're calculating the price (as in what 30/60/120, 15/30/40 etc) refers to but using your sample, here's an example of calculating it on a page:

HTML:

<select id="dressType">
<option value="0">Dress 1</option>
<option value="1">Dress 2</option>
<option value="2">Dress 3</option>
</select>
<label for="dressDays" size="3"> for # of days:</label>
<input type="number" value="30" id="dressDays" name="dressDays">
<button id="calcBtn">Calculate</button>
<p>Price: <input type="text" size="6" id="priceValue" value="0" readonly></p>

JS:

document.getElementById('calcBtn').onclick = calculate;
function calculate( evt ) {
// get selected dress
  var dress = document.getElementById('dressType').value; 
  var days = document.getElementById('dressDays').value; 
  var price = 0;
  
  switch ( dress ) {
  case "0":
    price = 60 + ( 15 * days );
      break;
    case "1":
    price = 90 + ( 30 * days );
      break;
    case "2":
      price = 120 + ( 40 * days );
      break;
    default:
      price = 0;
  }
  
  document.getElementById('priceValue').value = price.toString();
  
  console.log( 'Days, Dress, Price', days, dress, price );
}

JSFiddle Example

It's just using a switch statement instead of a series of if statements but either is fine. I used a switch statement since you appear to want to avoid the braces. The calculate button has a click event bound to the calculate function to produce the result in the price input. Hopefully it does what you want or leads you down the right path. Sorry for the late reply.