Flat Rate postage for multi items

Hi, am from Australia and our postal service have just started with flat rate (set rate) boxes.

With this, if you have 1 item or more items, if they all fit in the box, you only pay that flat rate ($8) anywhere in Australia.

We sell succulents, pots and accessories for doing succulents. I want to have the flat rate postage for when we post out succulents (have made a category for succulents under plants)

How can I setup shipping so if a customer buys 2 or more plants, the shipping they pay stays just $8 - the setup now is each plant is $8 per item, so if a customer buys 3 x plants, the total postage will be $24

Thanks, Sean