Shipping by total weight plugin. How do I configure for a fixed cost per 18 kg box?

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:

1 * 18kg box costs £10 shipping
2 * 18kg boxes costs £20 shipping
3 * 18kg boxes costs £30 shipping
3 * 18kg boxes + 1 * box less than 18kg box = 4 * £10 = £40
etc...

Hi,
You can refer to NopCommerce documentation https://docs.nopcommerce.com/user-guide/configuring/settingup/shipping/providers/manual/index.html to set up the shipping rate.
You would probably only need to define the weight of the product on the product detail page and need to specify the rate per weight unit in the shipping configuration.

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".

Can I configure this plugin to calculate the shipping as:

£10 for up to 18kg
then £10 for each additional 18kg?

Yes

As per the documentation:

... 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

Thanks. But what's not clear is the size of the unit of weight. In my case it's every 18kg or party there of.

In the end I gave up and modified the plugin myself to do it. This is a great thing about nopcommerce. Keep it up!