Page-sequence vs page-counter

Hello list!

I'm trying to create a PDF with invoices. Each invoice can have  
multiple pages and I would like to display the number of pages per  
invoice. So a pdf can have 5 pages, first invoice with 3 pages,  
second with 2. The first should have 1/3, 2/3 and 3/3, the second 1/2  
and 2/2 as page numbers.

I have added a simple page counter, but it doesn't work properly. I  
get 1/3, 2/3. 3/3, and then 4/5 and 5/5. I probably can find a way  
around this by substracting the total amount of pages for previous  
invoices, but I hope it can be done easier.

If I search for solutions I find references to fo:page-sequence. But  
if I try to do something with it, the PDF becomes invalid. So I  
wonder how I can solve this. I probably don't understand page- 
sequence properly. I think I can solve my problem if I understand how  
it works.

Thanks for your time!

Roger