At/in this section, you must enter your shipping details.
Should I use at or in?
They mean slightly different things. "In this section" means that the object is a point or space contained in the section. A quote is in the section (chapter) of a book or a book is in a section of a library. "At this section" means something that's near but outside the section, or applies to the whole section. You might have different lighting at the section of the library that a book is in.
One last note: sometimes, if the distinction is very small relative to you, the two are used interchangeably. For example, if you're planning to meet up with someone and you're across town from the the meet-up location (e.x. a library), you could use either in or at said library. It wouldn't be the most precise or helpful, but I've seen it frequently.
Certainly go with "In" this section. This is if you're talking about filling a form or some document.
But if you were to use 'at', it could be like this: "at this point".
However, they're not the same because 'point' would mean a location (as in the spot within an office) or moment of time.