Add that it is not an issue at this time, however, if it should become an issue as you make your decision, you will call to discuss it at that time. Their response may telegraph whether or not the offer is negotiable. You should always inquire about the company's salary review plan. Some organizations may grant increases after relatively short probationary periods or may have six months reviews which add to first year income. Conversely, depending on your starting date, your first year consideration for a salary increase may be longer than twelve months.
Some companies guarantee a percentage of overtime in addition to base salary.
Usually such a guarantee is spelled out in the offer letter but occasionally may not be. While this may add to your income, it should not be folded into your expected annual income unless it is guaranteed. Often in smaller companies, there are alternate types of compensation that may offset somewhat lower starting salaries. For the most part, benefits such as these are more common with experienced candidates going directly into management positions. If you have more than one offer, you should evaluate each one in depth before you make a decision.