Your boss is wrong, and this isn't a matter of interpretation. The law guarantees you're entitled to a break even with 6 hours of continuous work - there's no 8-hour threshold in Italian labor law. What changes is the *length* of the break: with 6 hours you generally get a shorter break than someone working 8 hours, but there still has to be one.
It's a shame this happens so often in clothing stores - they keep you there without a break claiming "we're not obligated anyway." I've seen situations like this myself, and what's frustrating is that a lot of workers don't realize they can say no. The law is pretty clear: the right to a break exists when the workday is long, that's it. Check your sector's collective labor agreement (the CCNL for retail in your case), because it might spell out the times better, but the legal foundation is there regardless.
Don't let yourself be rushed by "but we've always done it this way." If your employer keeps denying you a break, you can report it to the labor inspectorate or reach out to a union - it's not a serious thing from a bureaucratic standpoint, but it's a real violation. In the meantime, just start communicating that you need to take a break, even just 10-15 minutes. They'll have a hard time stopping you if you do it calmly and reasonably.