You need something from the couriers in a formal letter stating how they handled parcels, tampering, chain of custody etc
Even when using a transport carrier, LTL is a real thing for most companies as only the large companies get to ship full truck loads that i think SQF over looked
And because of this, LTL always caused issues in my past audits. Had an auditor ask me why our shipping checklist for LTL didn't include checking for hazardous chemicals on the truck. I've looked several straight in the eye and said "because unless I see spills or other direct problems, we're loading that truck." LTL carriers often use open air cross docking (like SAIA in my town right up the street), and they're mixing and comingling all types of pallets of product to put it on whatever truck is going to whatever destination. Pretending like I need to monitor what's on that trailer is a joke when they're going to put it next to everything under the sun at their hubs.
And zero LTL drivers are going to let you throw a seal on their trailer when they've got 10 stops after your plant. Best you can do is add marked shrink wrap or security tape to your pallets that let your customer see whether a pallet was rewrapped or opened while in the custody of the carrier.
And I'll second what Scampi said on letters of guarantee from your parcel/LTL carriers. Ask them for copies of their security practices, etc., and place some type of security tape on your boxes that are going via these carriers. And make sure your programs reference these security steps you're taking. It's less than ideal, but it's a reality we all have to find ways to live with.
