To update this post for others...

It seems that the problem in Ken's case is that his specification was using spec codes for the GL Account and Department (D13, D14) in the Header section, rather than in the Detail section for which the codes were designed. Thus, unexpected results occurred.

Using these codes in the detail section gave the expected results. They cannot be used in the Header or Footer areas.

The AR Customer Control Account can be inserted into the Header section, although this does not print a specific GL Account/Dept.

Alternatively, if there is one specific Account/Dept that is used for a particular Customer at time of entering the details of the invoice, an Optional Field could be used to keep track of this information. You would then enter the GL Account/Dept into the Bill To screen, in the identified Optional Fields on the bottom right of this screen.

You can then insert the spec codes E18 and/or E19 for Optional data 1 & Optional data 2 to have the information printed in the Header section of the invoices.