[skeoh]

The use of the word "hidden" in the article is misleading. The field is visible and editable by the user -- it is just styled in a way that does not make it clear it is an editable field. There is a GIF in the article showing the author changing this field value manually.

[edaemon]

It's not misleading at all; it's an input with the type set to "hidden". The field doesn't become visible until you select the radio button for that subscription level.

  <input name="amount" type="hidden" value="15000">

[skeoh]

Ah, I misunderstood. Yup that's pretty bad then!