Sean O'Grady: Forget the numbers. Think of the grannies

"The cheque's in the post" will soon be a more legitimate excuse.

The postal strikes will indeed cause a good deal of disruption in the run-up to Christmas, delaying the delivery of goods, invoices, cheques and contracts.

The hardest hit will be smaller companies and those who sell online. The Federation of Small Businesses says that 80 per cent of its members rely on the Royal Mail. If they need working capital, the banks can be relied upon to say No; the credit crunch goes postal, you might say. Online retailing is, despite the hype, not as extensive as is often assumed, and one-fifth of it comprises grocery deliveries by the big supermarkets, outside the scope of the strike.

At most, around 4 per cent of total retail sales might be affected: if a postal strike was so extensive that it knocked out such commerce between now and Christmas, it would cost £2.4bn in lost sales and reduce GDP by 0.2 per cent – a significant hit just as the economy is struggling to return to growth.

However, the impact is likely to be far less. Retailers such as Amazon are reducing their reliance on the Royal Mail and many online sales will simply go "offline", boosting book shops and music stores instead. Items sold on eBay will migrate back to car boot sales.

The decline of the cheque in favour of direct debit also reduces the impact compared to previous eras. In 1971, a two-month national strike crippled the banking system (the posties lost). Not so nowadays: A mere 0.3 per cent of all bills are paid by post. Some individuals and businesses may incur additional bank charges, which paradoxically could boost GDP, but the net impact would be small. Vital contracts and materials would be more costly to lose or delay.

The third impact is more nebulous – on confidence. Just as the economy seems on the brink of a fragile recovery, it could be argued, the last thing we need is another dose of morale-sapping industrial militancy.

As always in economics, the strike will make a difference "at the margin". It certainly makes a perfect excuse for the apathetic to skip sending Christmas cards this year. There may be some very disappointed grannies out there. So never mind the modest economic damage – how can we put a price on such emotional pain?