Clear this up for me as I'm not clear on it.

I walk into a burger joint and pay $5 in Bitcoins for a burger. Let's presume this is the near future and I can do this with my smartphone.

Now, the transaction needs to be approved in about 3 seconds. You can argue with me about this until we're both blue in the face, but let's not. Let's just accept, for purposes of this argument, that it reasonably should approve within about 3 seconds.

Let's also assume that there is no such thing as just assuming it's good because it's an inexpensive transaction. That doesn't work for me either. If a business assumes that, then I guarantee they'll stop assuming that as soon as a set of regulars take advantage of that somehow.

Final assumption: the burger joint doesn't really care whether they collect $5 or $4.90 or some figure close to that. And the customer doesn't really care whether the burger costs $4.90 or $5 or something close to that.

Therefore, it seems reasonable that the burger joint can tack on an extra fee to the transaction (they need not state it like that though). They just advertise burgers for $5 and eat a 10 cent transaction fee.

So, my question is, can a transaction fee in the realm of 10 cents or so on a $5 transaction get this transaction approved in 3 seconds or less? If so, great! If not, then that's not so good.