The idea is that the Post Office will have a budget for 'losses in transit' and they can just pay the claim and the accountants will just transfer the loss to that budget without affecting the real business. Similar ideas are that 'you can write it off against tax' if you claim [the expense] as a deduction from your taxable income or 'you can write it off agaianst insurance' if you include a small unjustified claim (like this one) when you make a large justified one.

The idea doesn't really bear close examination (as Jerry demonstrates) but it is a popular phrase nonetheless.

The idea is that the Post Office will have a budget for 'losses in transit' and they can just pay the claim and the accountants will just transfer the loss to that budget without affecting the real business. Similar ideas are that 'you can write it off against tax' if you claim [the expense] as a deduction from your taxable income or 'you can write it off against insurance' if you include a small unjustified claim (like this one) when you make a large justified one.

The idea doesn't really bear close examination (as Jerry demonstrates) but it is a popular phrase nonetheless.

Edit: after consideration, I think it all springs from 'write off against tax'. It is true that sometimes a company will, for example, keep a loss-making division going so that it can claim the losses as a business expense, and pay less tax because of it. In such (rare) circumstances, the 'tax write-off' has a value of its own. The concept, however, has spread beyond the rarefied accounting and legal circles where it has a genuine meaning and into general use where 'a write-off' (not to be confused with the sense of a vehicle damaged beyond repair, though the origins are connected) means merely 'a loss that's trivial or unreal'.