A packing slip has exactly one job: confirm that what's supposed to be in the box actually is. Everything on it should serve that single purpose.
Order and shipment identifiers
Order number and, if relevant, a shipment or box number (for multi-box orders) let the recipient match the slip to the right order without confusion, especially useful for warehouses receiving several shipments at once.
Sender and recipient details
Company name and address for both sides, similar to what you'd expect on any shipping document — confirms who sent it and who it's meant for.
Itemized contents, with quantities
Each item, described clearly enough to identify, with the quantity included in this specific shipment. If an order is split across boxes, each slip should show only what's actually in that particular box.
A tracking number, if applicable
Including the courier tracking number on the slip itself (not just in a separate email) gives the recipient one less place to look if they need to check delivery status or file a claim.
What deliberately isn't included
No pricing, no payment terms, no invoice-related details — those belong on the invoice, not here. Keeping the packing slip focused purely on contents keeps it doing its one job well.