Dear Chinese Food Delivery Man,
Thank you for being so friendly. You're always very nice. However, I really feel that you crossed a line today. While I know you must think that you were being very helpful, your unsolicited "Would you like a good way to lose your weight?" was unwelcome and, frankly, inappropriate.
Then, you kept me at my door for a good 3 minutes giving me weight loss advice (and poor advice at that.)This was very uncomfortable for me. I couldn't just close the door, because that would be rude--and you were being unintentionally rude enough for the both of us. Also, sir--and this may come as a shock--but you are not telling my anything that I do not already know. You see, I know that I have a weight problem. This is not a new thing. I have always struggled with my weight. I am currently seeing professionals to help deal with my physical and (severe) psychological issues around my weight. You have just severely enhanced the latter with your "help."
Please be more sensitive next time,
Your Upset and Highly Embarrassed Customer.