Yes!
During your share customization window, simply increase the quantity of an item that you would like to receive more of (assuming that there is no limit placed on that item).
Each box has a base value of $45. When you add extra vegetables to your share beyond the standard $45 box, you will be charged for those extra vegetables.
When the share customization window closes for the week (usually on Thursday mornings), your credit card on file will be automatically charged for the extra vegetables that you added to your upcoming delivery.