BruceM wrote:
I am inclined to use autonumber or some other fixed value (e.g. Invoice
Number) as the PK.

Word of warning: an incrementing autonumber can leave small gaps in a
sequence e.g. a autonumber value was served out but the transaction was
rolled back and the served number is never reused. I've heard said that
if exposed, e.g. as an invoice number, you may have to account for the
missing numbers to an auditor. I know from a previous career that an
auditor will ask about missing numbers in a sequence of cheque/check
numbers. The only 'orders' database application I've worked on was
'life critical', hence we used no autonumbers and all data had to be
retained for audit trail purposes, even on rollback.