Hi team

I've just posted on the forum, a question regarding Flat Postage Rules for business sellers. I thought perhaps that I was doing something wrong with regards to using the rules as combined postage wasn't working as expected. It turns out that I'm not the only seller having this problem. Is there a glitch in the system?
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My original post:

I've set up a flat postage rule and applied it to all of my postage policies, assuming that, as per the highlighted text in the screenshot, buyers pay the highest postage fee, and each additional item is sent with free postage:

However, if I do a test purchase and place items in my basket which use different postal rates, it only seems to combine items which fall within the same rate. e.g. Large Letter or Letter. Based on the above, I would expect to pay one charge (the highest), and for each subsequent item to be free postage. Am I doing something wrong or am I misunderstanding how the flat postage rules work?

Here's an example of four items in my basket, two at Large Letter rate and two at Letter rate. I would expect to pay £1.80 flat rate for all four items, but instead am being charged £1.80 + £1.15:

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Thank you!