The "Required Supplements" are all those additional costs at the final auction price that the winner is obliged to pay.
Remember that there are products that may or may not have "Required Supplements". An example of these supplements is the tourist tax paid in some cities visited, shipping costs associated with the product purchased or the "Costs of handling the auction of physical products". As you can see, it is a supplement that, unlike "Optional Supplements", you can not stop paying.