Products
It can be specified whether a product uses the weight based breakpoints, where all weight breakpoints that qualify will be displayed on the shipping step, or a single quantity breakpoint can be assigned to the product instead. This is handled by a product metadata field called ‘Breakpoint Delivery Rule’.

Please be aware that if you have weight breakpoints that have a price assigned to a 0 unit measure, even if you have only quantity products in the basket, because the order weight is 0 the weight breakpoint will be displayed and if there is a cost associated with this unit break, this will be applied if the breakpoint is selected, so it would be advisable to not add a weight to the 0 unit, but add a 0.1 unit if you need to add a near 0 weight cost.