For high ticket items, I film myself packing the box. Not a fly on the wall thing – just my phone hanging camera-end off the desk whilst I pack and seal the box beneath.
I learnt this from a company who sell high-value computer components and often had disputes when people didn’t fancy paying.
I then take a photo of the box on a set of scales to record the weight. And then take a photo of any receipt from the shipping company – a lot of them will also post up the recorded weight when it gets to their depot also.
That’s about as much as you can cover yourself. Email it to yourself on GMail and then you don’t have to worry about keeping a backup or local copy, or stick it on a dropbox if you have one spare.
Only takes a few minutes, and is worth it if its any amount of cash you’d miss. It’s up to you how much cash that threshold sits at, mind.