GLS charges an index-based surcharge (in percent) 'Energy' on the parcel price, because fluctuations in energy prices (diesel, electricity and gas) have a significant impact on transport costs.
In calculating the percentage, GLS uses the 'Motorists Price Index Diesel', as well as the 'Producer Prices for Natural Gas Commercial' and the 'Producer Prices for Electricity Special Contract Customers'. These values are published monthly by the Federal Statistical Office.
After the index is published, GLS determines the percentage which will be used for the invoices issued from this point onwards.
You will find the current percentage here.