I phoned Venice San Niccolò:
opening hours 900-1300 and 1400-1900 local time (closed on Mondays)
fuel currently
unavailable until 25/9 fuel available after 26/9
If you are arriving/departing France/Germany/Austria/Slovenia no customs necessary: you only need to specify in box 18 of flightplan that pilot and pax have passports
http://www.enav.it/aip/aga/aga2-3/AGA3-117.pdf
If arriving/departing Switerzerland/Croatia customs can be done at Ronchi dei Legionari - Trieste or at Treviso
http://www.enav.it/aip/aga/aga2-3/AGA2-50.pdf
http://www.enav.it/aip/aga/aga2-3/AGA2-49.pdf
AE