The fact that you had people complain suggest that there is something wrong. Of course, it greatly depends who is doing the complaining, since some people will complain about all sorts of nonsense, but reading your example, I too have a complaint. My complaint is not that you have inline speech (I actually like inline speech, it makes the dialogue less dry, I see nothing wrong with that) it's that you have one character speaking while a different one is performing an action in the same paragraph.
"Two fifty," replied the cashier. Jill handed him the money, took the mints, and left the store.
The cashier is speaking here, but it's Jill that's doing the action. This isn't a very complex action so as to confuse anyone, but writing this way increases the chance that there might come an action complex enough that would make it hard to differentiate who is actually speaking. It's much more cleaner, so to say, if it's separated:
"Two fifty," replied the cashier.
Jill handed him the money, took the mints, and left the store.
Some even go as far as to separate any action made by different characters into their own paragraphs. For example:
Jill handed him the money. He took it, deposited it into the register and returned ten cents worth of change.
There is no dialogue here, but remember, dialogue is just another type of action. It will always be much clearer if each character gets his own paragraph. If this was Jim and not Jill handing him the money, could you be absolutely certain who is returning the change, Jim or the cashier? When you put a paragraph break there, that's a queue to the reader that a different character is performing the action now, so there's less chance for confusion.
Jim handed him the money.
He took it, deposited it into the register and returned ten cents worth of change.
Like others had said before, there are different styles used by different publishers, which change and evolve not only with time but place as well - different countries have different customs. And these are only customs, things readers are accustomed to from books they've already read. For example, in some countries you'll find dialogue separated by dashes instead of quotes, and while it won't confuse the readers from those countries because that's what they're used to, it will confuse readers from other countries. The one single rule we can all agree on is not to confuse the reader. Your best bet for not breaking that rule is to stick with what the reader is used to.
Also, don't ask yourself what are the rules of style, ask yourself how can you make things more clear to the reader. I think everyone can agree that separating different characters actions (be it dialogue or not) into their own paragraphs can only make it more clear, not more confusing. In other words, I see more benefits in separating it than leaving it in the same paragraph. Actually, I see no benefits at all in leaving it in the same paragraph.