George wrote:

That line item has been on the PA tax return as long as
I can remember. On occasion they have been known to obtain audit
information from other states and send out letters.

Correct. Be aware that state sales tax workers have a background
communication channel between themselves. If, during an audit of an Oklahoma
business, the auditor discovers the business made significant sales to
Company A in Ohio, the auditor makes a note to himself.

Later, the Oklahoma auditor snitches out Company A to his cohort in Ohio.
The sales tax entity in Ohio then swoops down on Company A.

Take away: If audited, don't open your records to the auditor. Show him only
those invoice copies pertaining to your state. Don't be a rat. (Besides, if
your customers ever find out that you were the one who gave them up, your
butt's busted.)