without seeing the actual part file (that you should attach if you want a meaningful
answer), one can just guess that either the tolerances you used for surface modeling or the STEP exporting tolerances are too tight.
Is the part file also that bigger (79 times) compared to your usual parts or is it just the STEP file ?
Can you copy your model to a different layer and do a split body on it? Cut it into 3 or 4 sections. Export each section separately. Your customer can import each one separately and unite them in their software. Try to split the model in areas where geometry is not complex. This will aid in uniting the bodies in your clients software.