SlipMatch
Two PDFs go in; one PDF comes out with each slip followed by its label, in packing order.
Pair your real slips and labels, see the full packing order including anything it could not pair, and print the first 10 orders.
$5 a month on its own, or in Documents ($12 a month); every paid tool for $24. It adds: Larger batches than the free tool handles.
One subscription, one invoice — add a tool any time, prorated. Sign in and it unlocks; or paste your key once per device.
Email support@avyth.com and the person who built SlipMatch reads it. If it fails on your file, say what the file looks like — that is usually enough to fix it.