When a parcel's import duties have not been paid in advance, there are additional costs, paperwork and delay times to the delivery of the parcel.
VAT% + fee per parcel ~ 2 day delay
It a very bad surprise for most customers as it feels like they did something wrong or their parcel has been taken hostage by the delivery service and they have to transfer money to a 3rd party.
Having the cost upfront, included in price, shipping or a backerkit addon called "EU-Friendly shipping" would greatly increase trust.