It's a painful aspect of importing. Items that go through the postal system quite often don't get held up.
Unfortunately, it's not always obvious that an item will arrive through ParcelForce. So while the shipper may think they are doing you a favour using 'express post' by the time it gets here, it becomes a ParcelForce EMS item. It might be difficult for the shipper to even establish whether you'd end up with the higher, lower or no charge.
What really annoys me about it is that they WRITE to you to ask for the charges. This can take 2-3 days and then another day to get it out to you. They do this even if you have an account! Even UPS will just take a cheque at the door (but then they charge £20).
In fairness to ParcelForce, we get a pretty good service from them for UK and European deliveries.