Just guessing, if you go to the table where the related items for the client reside, are they in the correct order? Do the line items show up as a portal on the invoice? And how is that set up to sort?
Some screen shot may help
I would guess that like the starter solutions that come with FileMaker, this database uses a script to pull up the line item records on a print layout based on the line items table instead of the invoices table. This script may containe a sort records step. If so, you can find and open up this script to learn the sort order specified and you can change it. If it does not have a sort records step, it is quite possible that there is no sort order specified and then the items will be listed in the order in which they were created--which may not match the portal to line items on invoices if there is a sort order specified for the portal.
But not having a copy of this solution, I am only guessing as to likely reasons why you are getting a different order to the line items.