"It'll only be bad news"
You treat the drawer as though it were full of bad news. That's why things go into it. Let's look at what was in the one from chapter 1, since I'd guess yours is similar.
There were eleven envelopes. Two were bank statements, showing things that had already happened. One was a pension update, which needed no action. Three were marketing, which included a loan you don't want and a credit card you don't need. One was a notice of changes to terms and conditions. One was a cheque for eighteen pounds. That's eight out of eleven that were harmless, dull or good.
The other three were the insurance renewal and two letters about the parking charge. Even those weren't bad news in themselves when they arrived. The renewal was a price and an invitation to talk about it. The first parking letter was a thirty-five-pound nuisance. They became bad news in the drawer.
I can't offer you a study of what's in people's unopened post, so this is my own reasoning. But I'd suggest that for most people who aren't in serious debt, the proportions are about like that. Most money post is routine. It's statements and confirmations, annual summaries and marketing. The post that matters is a minority, and even that, caught on arrival, is mostly small stuff with a number and a date attached.
What the drawer holds in your mind is something else. An unopened envelope has no size. It could be anything, so it becomes whatever you most fear. It's a demand for thousands, or the account is being closed, or you've been found out in some way. You feel a small clench when you see it, and the clench is a response to the worst thing it could possibly be. Eleven of them together produce a steady low dread out of all proportion to their contents, which in the example came to a hundred and thirty pounds of avoidable cost and eighteen pounds of good news.
When you open an envelope, a dread of unknown size becomes a number of a known size. That's nearly always an improvement, even when the number is unwelcome. You can plan around a number. You can ring someone about it or pay it. You can dispute it, spread it out or put it in the diary. With a dread you can't do any of those things. All you can do is avoid it, and avoiding it keeps it alive.
Think back to the last time you did open something you'd been dreading. What was it like in the minute after you read it? I'd bet on one of two things. Either it was nothing much, and you felt foolish for the weeks of worry. Or it was a real problem, and alongside the dismay there was an odd relief, because now at least you knew what you were dealing with. People say this about all sorts of bad news. For many people the not knowing was worse than the news.
The honest objection is that sometimes it really is bad. Sometimes the envelope holds a bill you can't pay. Chapter 7 is about that, and chapter 14 goes further. For now I'll only say that the bill you can't pay is the same bill whether you open it or not, and the only difference is how many options you still have. Those run out with time, as the last chapter showed.
The rest of the time, which is most of the time, the envelope holds something dull or a small sum with a date on it. You've been dreading a drawer full of dull things and small sums.