Saying yes and no

Clearly saying yes and no is hard. Have you ever been caught out in a question like this:

You don't want me involved, do you?

If I say, "Yes" am I saying I don't want them involved or the opposite? Usually to make it abundantly clear in English I'd answer,

Actually I'd like you to be involved.

or

No, I think we'll be fine without you this time. 

It is worse in Japanese and I still haven't figured it out.

This morning at the grocery store I said to the cashier literally "the bags are okay" 「ふくろはよろしです」, meaning that I didn't need plastic shopping bags with handles and she understood me (I've practised this many times and this particular phrase always works). Then she asked if I wanted plastic bags to cover just my meat packages. I didn't know how to answer, so I said "hai" which sort-of means "yes" (but isn't the only way to say yes and often is used as in "I hear you" in continuing a conversation) but it was obviously wasn't the right yes today. She didn't give me bags. Maybe her question was, "Are you okay without bags for the meat?" And I agreed with her?

I feel bad at Japanese 95% of the time. At times like this I feel like I'm failing even the easiest of tasks. It wasn't a terrible error by any means, but I failed to communicate what I really wanted, and that was bags to cover my meat so it didn't leak on my cloth bags (though the meat was already technically sealed, I find the plastic wrap doesn't always do the best of jobs here).

Perhaps someone better at Japanese than me can explain what I should have said?