You have to pay.
For me, as a seller of low cost items, that is one of the issues of Simple Delivery. Previously, you were able to make a small charge, included in the postage cost, to cover the cost of a padded envelope, bubble wrap and tape etc.
Now, that is not possible and private sellers have to stand the cost of any packaging they use.
Personally, I am not using Simple Delivery until I absolutely have to.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)