I was recently taken aback by the prevalence of round-ups for charity when making electronic payments. Just last week, it happened three times in one morning on the High Street, and even sneaky old online retailers are slipping it into their payment processes.

It works like this: if your purchases cost £7.84, the screen asks if you’d like to round up the bill to £8 in the name of charity. McDonald’s, for example, do it at their self-service screens, and Costa occasionally spring it on you through their card machines. Tesco have been known to, ahem, just do it.

The theory behind it is multi-layered, and it’s clever.

In this mode, the act of donating is wrapped up in secrecy. Nobody is going to feel guilty about offering their coppers, and nobody sees precisely how much is hoofed into the kitty.

Added to this, punters give more regularly because it doesn’t feel like giving. This is why restaurants pop up the tips question onto the screen of the card machine. It is a shallower dig than the one into the pocket for cold, hard monnaie.

Moreover, resistance to forking out is further weakened by the relatively meagre amounts involved. With mere pennies, customers might be less inclined to hesitate to contribute.

The collector therefore snaffles smaller donations but on a significantly larger scale, which tot up to a considerably more imposing pile than the one they’d trouser, had they asked for ad hoc donations.

This MO – applied nefariously – is a well-honed scam known as salami slicing or penny shaving. Add a couple of cents onto every transaction, which nobody will notice, and watch it the lucre mount up.

Incidentally, banks in the UK have used a similar method beneficially in order to encourage customers to save, by artificially rounding up transactions but (consensually) diverting the pennies into customers’ savings accounts.

With this charity caper, the collecting organisation ends up writing a meaty cheque to the cause of their choice (not yours), and everyone’s a winner.

Everyone except the customer, that is.

Every donor will have chipped in to give both the charity and the business a shot in the arm while still stumping up top retail dollar for whatever they’re carrying away in their 10p bags.

It does rather beg the question why the companies don’t just fork out a hefty wedge themselves for the char-i-dee, which would of course be tax deductible. Isn’t that the universally accepted trade-off with commercial charity support?

The answer is of course that if the customers suck it up, that trade-off looks commercially much healthier. The business gets to slap their name on a huge cardboard cheque without having to part with a rounded-up penny.

That’s having the baby without the labour pain.

So, the takeout is this: there’s nothing wrong with charitable giving, but it’s as ever always recommended to choose your channel wisely.

It’s probably sound advice to seek out a collector for your favourite charity and then just stick a couple of coins into the tin. That might then prevent any hovering charlatan from undertaking a financial or a reputational skim.

After all, give them half a yard, and they’ll round it up to a whole one.