And you honestly think that if you just said something else, they won't believe you?

Or are they going to open up the parcel to check it?

Frankly however, it's none of their business as they are just a collection point and nothing more at the end of the day.  The post office is not part of Royal Mail.

And not every item is sent through the post office anyway.  I would imagine that if a large/heavy item, it would have a collection arranged and I have never known the posties to even question it.

it's scan and put in the van.