To be aboard for another couple of international voyages, or not to be? That is the question travellers face on the ferry between Elsinore – home of Hamlet, Prince of Denmark – and the Swedish town of Helsingborg.

Barely a couple of miles of Baltic separates this pretty pair of ports, yet this strip of no-man's-water proves profitable for HH Ferries (00 46 42 19 80 00;

Since the international voyage takes only 20 minutes, I was intrigued to discover a full-service restaurant with an elaborate menu.

It turns out that anyone who orders a meal is free to shuttle back and forth as often as they wish on a single ticket, taking in the handsome townscapes while dining on first-class seafood and sipping cheaper beer than on land. The ferry firm would have been even happier had I spent a couple of crossings visiting the on-board supermarket for cut-price butter, salami and perhaps a pack or two of Hamlet cigars.

Price disparities fund all manner of quirks in the travel industry, but not all of them are as above-board as what I shall call the Hamlet hoppa. Smuggling and holidays are hopelessly entwined. Plenty of travellers have sunk a (duty-paid) pint at the Smuggler's Den in Newquay, or enjoyed a night at the ex-smuggler's den-but-now-VAT-registered George Hotel in Rye. But some folk use holidays as a cover for bringing back souvenirs they shouldn't.

Travel firms have little control over what their passengers get up to, and in particular what they put in their luggage. Which means a new fly-on-the-wall TV series about Customs officials could become compulsive viewing – and make travellers a little more wobbly when walking through the Blue or Green Channel.

Customs UK, to be aired on Sky Real Lives from 5 November, was filmed at the airports at Gatwick and Bristol, and at Dover Western Docks. The programme allows you to watch the people who are watching the smugglers who seek to profit from porous frontiers when prices differ sharply between one country and another. A packet of 20 cigarettes in the Canaries or Turkey (both outside the EU for customs purposes) costs about £2, one-third the price in Britain. That kind of divergence is enough to persuade some of the three million or so British holidaymakers who visit those destinations each year to exceed their duty-free allowance of a single carton (200 cigarettes); in one programme, a man flies in from Turkey with no fewer than 25.

More serious still is smuggling stuff that is illegal: weapons, pornography and drugs. According to the statement released by HM Revenue and Customs last Sunday, some travellers have taken the traditional "booze cruise" to the next level.

"£1.2m coke cruise" was the headline. "Seven people were arrested today in an early morning raid of the P&O cruise liner, Arcadia, in Southampton. Four of those arrested had a total of approximately 30kg of cocaine strapped to their bodies." Even by the standards of the weight-gain many cruise passengers experience that seems extreme. No crew members were involved; a spokeswoman for the company said, "This is absolutely the first incident of this kind involving P&O cruises."

Millions more British travellers, unwittingly or otherwise, break the permitted limit for overseas purchases of £145. Customs officials regularly conduct sweeps on flights arriving from New York, Dubai and the Far East. Some old lags ditch the receipts for their new clobber before coming back through Customs, and even snip the labels out of clothes to make their provenance harder to determine.

Others are genuinely appalled to learn that retail therapy on Fifth Avenue or along Singapore's Orchard Road can land them with a hefty bill for duty and VAT.

From 1 December, though, life becomes easier for big spenders. While duty-free allowances for tobacco, alcohol and fragrances stay the same, the limit for "other goods" is to rise from £145 to £340 – just in time for that Christmas shopping trip you promised yourself.

The only bad news about HMRC's festive gesture: when Europe's finance ministers originally adopted the higher limit last December, the pound was so strong that you could have indulged yourself in Boston or Beverly Hills to the tune of $700 without breaking the limit; this discontented winter, make that less than $550.