I disagree, been with DVDSoon for a a couple of years and it's the first time they've done it to me.
It may be more likely but as they pay the postage for card holders, not entirely out of the question as they would be saving money.
You should be able to claim some of that money back, check out the Customs/Importing/Charges page, someone on there managed to get some money refunded as the price on the package should have been the discounted price and not the price DVDSoon wrote as the value.
I remember ordering 2 GC games that I paid extra for part shipping so that they wouldn't be shipped together. However, they shipped them both meaning the pack was worth £52. Lucjily though they marked down the pack to $22 Cdn, or £10. Recently had RE4 delivered that cost £28 and was marked as such. Must be lucky as I have never been caught so far and have had 30 seperate packages, (40 items), sent by dvdsoon. Plus others sent from USA, Australia, Hong Kong etc. basically, the risks are small, but they are there. If you buy a lot the odd charges that you pay are insignificant to getting items months early and cheaper than in the Uk.
All my orders over the past year (free shipping time period only) came in one delivery. 2 or 3 dvd's in each order. Except the last one that has taken about 3 months to deliver a total of 3 dvd's. Always kept the order under £18.
I suppose they might have just split the orders over £18 knowing of the custom regulations for UK. Can't blame them for stopping this if that's the case. Easy enough for the customer to order each item seperately.