I don't know about carseats either, but $50.00 is too much to budget for that short a cab ride. Mary, Queen of the Universe Shrine is very close to the entrance of WDW. I would guess at $12-$15 each way tops. If you go on a Sunday there will be little or no traffic and it would be a very quick ride. I understand that Mears offers shuttle service from the WDW resorts for all the Masses except for (I believe) the 9:30am Mass.