Create Rates for Shipment Groups

I have equipment that is loaded and delivered to the customer several times a day. This equipment has a flat rate for one day, e.g. 400€. However, the costs are divided depending on how many THUs were loaded. e.g. flat rate 400€, 17 THUs fit on the equipment. In one shipment 6 THUs were loaded. The customer pays 141€ (400/17 * 6). This flat rate is calculated per day, i.e. if I load the equipment a second time with 4 THUs, the costs are 235€ (400/17 * 10). My idea was to combine all shipments that were delivered with the