What not looking costs
You've met most of the costs by now, but it's worth seeing them in one list. Individually they're small, and that's how the habit survives. Nothing on the list is dramatic enough to force a change. The total is another matter.
What follows is my own account, as observation.
You pay penalties and lose discounts. These are the clearest, because they're itemised. Chapter 3 described a parking charge that triples, a tax penalty that grows by the day, and the right to pay council tax by instalments being lost. Add the late fee on the credit card and the charge for a bounced direct debit. It's all pure waste. Money leaves your account and you get nothing for it.
You overpay. Insurance renews at a higher price when a phone call would have lowered it. Fixed deals end and revert to standard rates. Subscriptions carry on for things you don't use. None of this is a penalty. It's a toll on inattention, and companies are well aware that a share of their customers never read the letter.
You miss things that were in your favour, like the cheque from the water company, the refund that needs claiming or a tax rebate. There might be a letter saying you've been overpaying and asking where to send the difference. People who don't open post assume it's all demands. Some of it is money for you.
You miss mistakes. A payment might be taken twice, or a direct debit might not stop when you cancelled it. There could be a charge you don't recognise. If you never look, these run on for months.
You pay in the currency of chapter 6, which is dread. The drawer produces a steady low hum. You feel it when you walk past, and when the post arrives, and when someone at work mentions their mortgage. It costs sleep for some people. As that chapter argued, you carry this whether you look or not, and looking is the only thing that ever reduces it.
You pay in moments like the checkout. There's the declined card, and the bounced rent. There's the call from a number you don't know, which you let ring out in case it's about money. These are small humiliations, and each one makes the subject a little more painful and the next look a little less likely.
This is the pattern the series keeps finding, and I'll state it without borrowing the language of addiction, because nothing here is a craving. When you drop the letter in the drawer unopened, you get a small, immediate relief. That relief rewards the habit. Meanwhile the thing you're avoiding grows, in pounds and in dread, so there's more to avoid next time. The habit relieves a discomfort and builds it up with the same action.
You may pay in trust at home. If you share finances with someone, your not looking is their problem too. Perhaps they don't know about the drawer. Perhaps they do, and have given up raising it. Perhaps you both avoid it, by silent agreement, and neither of you knows the number. Money kept in the dark between two people tends to come out eventually, at a worse moment than any you'd have chosen. Chapter 14 has a little more on this.
And you lose choices. Someone who knows their number can decide things. They can take the holiday or not, change job or not, help their daughter with a deposit or not. If all you have is a rough idea, you have to hedge everything. You might say no to things you could have afforded and yes to things you couldn't, because you don't know which is which.
Set against all this is one benefit, which is that tonight is slightly more pleasant.