your select list must be named the same as the original text box, the name for the text box was "proResCart_1_Quantity_Add":
<input type="text" name="proResCart_1_Quantity_Add" value="1" size="4" >
your select list must use the same name:
<select name="proResCart_1_Quantity_Add">
instead of:
<select name="quantity">
the options do not get a name attribute, only the select element. Options only get a value attribute.
change:
<option name="ProResCart_1_Quantity_Add"value="1"selected= "selected">1</option>
to:
<option value="1"selected= "selected">1</option>