My products are typically sold in 18kg boxes. How do I configure the FixedByWeightByTotal plugin to calculate cost for each 18kg (or part) of the total weight:
Note that the Fixed or By Weight will use the total cart weight unless you mark each product as Ship Separately, in which case each item (and quantity) is a separate request to lookup the rate, and they will be summed for a single rate shown to customer.
In either case, you enter From and To for a weight range . In your example, the total cart weight should be fine, since your are charging the same rate £10 for "box less than 18kg".
... formula [additional fixed cost] + ([order total weight] - [lower weight limit]) × [rate per weight unit] + [order subtotal] × [charge percentage] to calculate the fees, where:
additional fixed cost - is the cost of shipment in case the weight is under a certain level (lower weight limit) rate per weight unit - is the cost of each weight unit above the lower weight limit