Calculating Courier Costs

Please could someone help provide a formula for me to calculate courier costs based on a consignments combined total weight?

Courier charges are tiered in 3 levels and are calculated on the combined weight of all boxes.
A flat fee for the first 20Kg of £6.25,
and then a cost of £0.28 per kg up to 100Kg
then a cost of £0.35 per Kg for anything above 100Kg

I have attached a basic spreadsheet as an example.


Thanks

Dan