sounds like what you are looking for is blank space on the page...just wrote an invoice myself.  Put the bottom part of the page in the page footer (which always print at the bottom).  Now if you have lines make sure that the lines touch the upper boundary and in their properties select extend to bottom of section (or something like that)...looked great for me...depends on how you want your invoice to look and if it mimicing a printed form or is more free form like most computer generated forms.

I do all of my report layout in the designer, so none of my objects are added dynamically.  You can hide and unhide (conditional suppress) items using formulas, all that hey want is a true or a false.  You can move items from side to side (kind of).