How many stamps do I need for this?

I’m a little confused as to how many stamps I need to mail this envelop.

It’s a Ready Post Utility Mailer envelope (roughly 15×10.5 inches). I’m sending a story to a magazine.

Last time I did this, I put the story in a manilla envelope, but the mail lady told me it should go inside a Ready Post Utility Mailer, which made sense because I didn’t want it to get ruined or bent or anything. I didn’t use any stamps at the time and I think she stamped it herself, which is what confused me. I paid everything there on the spot when I gave it to her. I never had experience mailing big things like this, just letters in envelopes.

So when I got my reply from the magazine, they told me next time to “self-stamp” the envelope. So. . that means they want me to just stamp it myself on the Ready Post envelope, right?

But then I calculate roughly how many stamps I need using the USPS’ website. The total price is like a bit over $5, so I’m like “Dang, that’s a lot of stamps.” I just wonder if I’m doing this right. . . .

And so does this mean that I won’t need to pay the mail lady if I put the right number of stamps on the envelope?