If you billed yourself out to an arm's length client as "engineer" in your title and billing rate, then you must have thought you were practicing engineering. This requires a license, regardless of what you are designing. All final engineering deliverables must be PE sealed per this NM statute worded in clear Anglo-Saxon prose:
16.39.3.12 SEAL OF LICENSEE:
A. Each licensed professional engineer shall obtain a seal/stamp, which must appear on all design drawings, and the certification page of all specifications and engineering reports prepared by the licensee in responsible charge.