Setting delivery costs at one set price for up to 50 items

[villawoz]

Hi there, I have a store that sells plants and shipping is £11 for 1 plant up to 50 plants.

So if the customer orders 1 or they order 49 it is still £11.

I made postage £11 but when you add 2 items it doubles it to £22 etc.

How can I set this up in Prestashop so that any quantity of up to 50 items is still £11 total shipping?

Any help most welcome.