Barcode on Dunning letter and invoice copy

Clients request to send the dunning letter and invoice copies to folder machine for auto folding into evelopes so that we will need to use barcode to group the pages. The barcode will consist with customer number and page number and total page number, e.g 12345 01 10 (where 12345 is the customer number, 01 is page number, 10 is the total pages). The question is, we are not sure that whether the dunning letter and invoice copy are coming from the same source. Are they able to calculate the pages in a sequential number.

E.g

Duning letter page 1 out of 3  

1st Invoice copy 1 page 2 out 3