is your issue related to your QC task of sending samples out to the 3rd party lab for testing? Does your job description only states the above specific job? Your job description does not include the generic "any other task assigned from time to time" statement?
Maybe your SQFP is asking you to fill out the corrective action form to get idea or assistance because he/she deemed or thought you might capable of doing this?
Usually corrective action plans are internal unless this is shared or required to be communicated back to the customer. You might not be in the position to respond directly to customer but a corrective action plan requires brainstorming to identify the root cause, the cliche more heads is better than one is applicable to this.
You sounded upset that you are being asked to do this. I would clarify my job description and training/ qualifications with my manager if you are not capable or refuse to do this task...