Are Birkenstock sandals (the normal Arizona style) acceptable for dress code standards at the basilica of St Francis ?
As sandals are a common choice of footwear for the Franciscan Brothers (among other monks, and Sisters as well) I don't know why they wouldn't be. :O)
https://www.flickr.com/photos/eliosk/19874790413
https://ro.pinterest.com/pin/504825439451501442/
More important would be wearing clothing that covers your knees and shoulders (the usual respectful attire for most Italian Catholic churches).