Yes, at STEP3 of the online booking procedure, under EXTRAS. Our partner SIXT ( for example) the following extras are available:-
WiFi Access: 12 EUR per Day Includes 13.5% Tax; On Request
GPS: 12 EUR per Day Includes 13.5% Tax; On Request
Infant child seat: 17.5 EUR per Day Maximum 87.5 EUR Includes 13.5% Tax;
Child toddler seat: 15 EUR per Day Maximum 75 EUR Includes 13.5% Tax;
Booster seat: 15 EUR per Day Maximum 45 EUR Includes 13.5% Tax;