Learn English – Best words to describe Food Expenses

In your first example, you could very easily replace "Sure" with "Yes "and have very similar meanings. All it's trying to convey is an affirmative response, and I would not suggest that it carries and implied Please or Thank you. Think of it as short for the following:

Would you like a cup of water? - I surely would.

In your second example, the sure is definitely not trying to imply that the appreciation is expected or required. Rather it is one of several words/phrases that have come to be commonly used in reply to thanks. E.g. no problem, certainly, etc. These all have a similar usage in practice to You're welcome.

Finally, while this usage of sure is informal, it's common enough that I'd feel comfortable using it all but the most formal scenarios, with the caveat that in situations like your first example, I'd probably make it Sure, thanks since I'm being offered something.

Generally, questions related to programming are off-topic here, and you'll get better answers over at the Usability Stack Exchange, but I'll take a stab:

From a usability perspective, the user shouldn't have to interpret the meaning of these labels. They should be obvious.

If I were you, I would think about labeling them with more than one word to make it crystal clear. Something like:

File
---New
------One-Time Purchase
------Recurring Expense
...