Use of the word "sure"? -- when offered something?

My friend and I feel differently about t use of the word "sure" in the context that someone if offering to do something for you. Perhaps it is idiomatic, depends on formality, or the closeness of the relationship. -- I think, if one asks "would you like some cookies", and they will be making the cookies, then you can answer quite comfortably with "sure!" -- and this will not be impolite. You are saying yes, but still allowing for debate on the issue. When the person actually delivers the cookies, you would then say "thank you." My friend finds the use here of "sure" to be impolite, somewhat like saying "whatever". The person should say something like "yes please" or "yes thank you". I think that we can use "sure" to imply that we want the situation to come about, however we say thank you when the situation is delivered. She thinks you should say "yes, thank you" right from the start. I think it sounds like a command, whereas "sure" invites debate to later leas to "thank you" Thoughts?