New online sellers sometimes assume this is just an oversight in their software, and add pricing back in without realizing there's a real reason it's left off by default.
The recipient often isn't the buyer
A package can be opened by a warehouse worker, a spouse, an office assistant, or a gift recipient — none of whom necessarily need or should see what was actually paid. The packing slip's job is verifying contents, not disclosing commercial terms.
Gift orders are the clearest example
A gift shipped directly to someone else with a price-inclusive packing slip spoils the gesture entirely — this single scenario explains a lot of why the convention exists and persists.
B2B shipments often route to different departments
A business receiving a shipment often has warehouse staff check contents, while a completely separate accounts department handles payment and invoicing — pricing on the physical packing slip would leak information to the wrong internal audience.
When showing a price might make sense
Some businesses intentionally include a "value for customs" figure on international shipments (for duties purposes), which is a specific, limited exception — not the same as full commercial pricing, and usually clearly labeled as a customs value rather than a sale price.
The safer default
Unless you have a specific reason to include a price (customs declaration being the main one), leaving pricing off the packing slip and reserving it for the invoice avoids awkward situations you may not anticipate until they happen.