The real problem is that you are trying to imitate a
handwritten paper invoice that had spaces for purchases and
the total at the end of the blank lines.  Other than old
thoughts about how it used to be done, there is nothing
wrong about the totals appearing after the last purchase.  
But, your company is probably not going to change its mind
about that, so you are stuck trying to munge your report to
have the traditional look.

There are a couple of convoluted approaches to this, but I
think using the Page footer is the easiest.  For each text
box in the report footer, add a text box to the page footer.
Make these text boxes invisible so they don't show on any
other than the last page.  Set their control source
expression to   =corresponfingreportfootertextbox

Now, to make this visible on the last page, you need a text
box somewhere in the report that refers to the Pages
property (the usual  =Page & " of " & Pages text box is fine
for this purpose).  The you can add code to the page footer
Format event to make the text boxes visible on the last
page:

    If Me.Page = Me.Pages Then
        Me.txta.Visible = True
        Me.txtb.Visible = True
            . . .
    End If