Physical letters do not obviate scams, nor is the cost that prohibitive. I remember actual 419 scams on blue airmail all-in-one letters back in the 80s. And that was international post too.

Think of it like changing your SSH port. It does nothing to prevent scams per se but you'll have to deal with only 0.00001% of them.

They don't remove it but they do reduce it.

I have an inbox, and I do not receive a lot of scam post. In fact, I don't think I received any since I lived at this address (~10 years ). We do get a few promotional leaflets every other week.

OTOH, I get hundred of spam emails every day.

The former is something which I can handle manually easily, the other is not.

It doesn't have to prevent the scam completely, it just has to make harder for them to scam you than it would be to move on to scam someone else.

If you are targetting a list of well-known authors I guess outsourcing the writing of a couple of hundred handwritten letters shouldn't be too hard. I'm sure they they can find a school class in Nigeria or Kenya who would gladly do it for a few dollars — or a struggling teacher willing to get creative with the homework assignments.