The "Optional Supplements" are all those additional costs that the winner of the auction has the possibility to do in case of having accepted the conditions indicated.
For example, during the "Night in a hotel in Porto with breakfast" auction, an extra night service may incur an additional charge that will be described as an optional supplement. In this case, for example, the Saturday evening stay would involve an additional cost of € 28.
Remember that there are products that may or may not have "Optional Supplements".