It's not straightforward doing this in  Word as one you probably have is
"multiple items per condition"  If you have Access, it would be better to
move the data into that and then use a report in Access which is better at
this sort of thing than Word.  One other alternative is to see the "Multiple
items per condition" item under the "Special merges" section of fellow MVP
CIndy Meister's website at

http://homepage.swissonline.ch/cindymeister/MergFram.htm

Alternatively, if you use a catalog (or in Word XP and later a directory),
type mailmerge
in which in the mailmerge main document, you have the mergefields in the
individual cells of a single row table and NOTHING else in the document so
that when you execute the merge you get a document that contains a table
containing all of the data, listed in customer order, then you run the
following macro
on that document, it will create separate tables for the each customer, one
table to each page.  Then with a bit more work, which also could be
performed by a macro as I think about it, you could get your invoices.

' Macro to create multiple items per condition in separate tables from a
directory type mailmerge

Dim source As Document, target As Document, scat As Range, tcat As Range

Dim data As Range, stab As Table, ttab As Table

Dim i As Long, j As Long, k As Long, n As Long

Set source = ActiveDocument

Set target = Documents.Add

Set stab = source.Tables(1)

k = stab.Columns.Count

Set ttab = target.Tables.Add(Range:=Selection.Range, numrows:=1,
numcolumns:=k - 1)

Set scat = stab.Cell(1, 1).Range

scat.End = scat.End - 1

ttab.Cell(1, 1).Range = scat

j = ttab.Rows.Count

For i = 1 To stab.Rows.Count

   Set tcat = ttab.Cell(j, 1).Range

   tcat.End = tcat.End - 1

   Set scat = stab.Cell(i, 1).Range

   scat.End = scat.End - 1

   If scat <> tcat Then

       ttab.Rows.Add

       j = ttab.Rows.Count

       ttab.Cell(j, 1).Range = scat

       ttab.Cell(j, 1).Range.Paragraphs(1).PageBreakBefore = True

       ttab.Rows.Add

       ttab.Cell(j + 1, 1).Range.Paragraphs(1).PageBreakBefore = False

       For n = 2 To k

           Set data = stab.Cell(i, n).Range

           data.End = data.End - 1

           ttab.Cell(ttab.Rows.Count, n - 1).Range = data

       Next n

   Else

       ttab.Rows.Add

       For n = 2 To k

           Set data = stab.Cell(i, n).Range

           data.End = data.End - 1

           ttab.Cell(ttab.Rows.Count, n - 1).Range = data

       Next n

   End If

Next i