Do it in a stored procedure not a trigger. Triggers have their place but a stored procedure is almost always better. Triggers can surprise you.

I don't follow how you would do that in a stored procedure outside of a trigger.

I think instead of performing an INSERT you call a stored proc that does the insert and some extra stuff.

Yes, we already have all of our business logic in postgres functions(create_order, create_partial_payment etc).

Doing the extra work in stored procedures is noticeably faster than relying on triggers.