I’m not quite understanding the problem here. First you say:
If I call the field in the custom section I get a result (15), so I know the field has a value.
And then you say:
I can give the shortcode a parameter for ID, but in that custom section, I can’t get the actual ID to put it in the shortcode.
Also, what do you get if you do var_dump($id_i_need)
?