I guess changing from grand child to parents may not be correct, as it is from the applicant's point of view.
The form also provides an example: "If your parent lives in Canada you should choose 'They are my parent or step-parent'".
Even I faced the issue of $170 as Bio metrics fee while applying for 1-person. However, when I changed the "documents to prove Child / Grand Child meets LICO" from multiple documents checklist to just 1 single document (NOA), the Bio metrics fee was updated to $85