[titchyyyy]

Could they launch an investigation once they receive the wrong item? The package is 40-50g heavier (out of ~2.5kg), although that includes the return papers and the tape used to seal it again as well as being a couple centimetres less wide. Should I be worried about this difference?

I wouldn't worry about it tbh. If your account is in good-standing, they won't even bother to check the contents of your box, never mind weigh it.

I've just returned an item that weighs 250g when the original item they sent was 350g - my account is not in the best of standing (30+ refunds) and they've refunded me, so.

If you become a 'professional' SEr like our Mothered, then little details such as the weight being within 1-2g of the original will mean the difference between getting a refund or not.

It's always good practice to get used to returning items that weighh very very close to the original item though, as you're practically guaranteed a refund every time with companies like Amazon.