Generous sister.
I once had to collect a customs fee for a birthday present from Portugal, all wrapped up in "Happy Birthday" baby boy paper. It was quite clearly a load of cartons of cigarettes. Customs people get to have a feel for certain "gifts." Many make it undetected due to sheer volume of parcels. Importing goods costs when done legitimately. Dealing with Customs charges incurred by shoppers costs Royal Mail time and money. We have to pass that on to the importer who has either not done their homework or decided to chance it. Some you win, some you lose.