Do i have to order a cake or can i just go and pick it up (sams club)?

k so on saturday i bought a cake,strawberry shortcake at sam's club.i am new at sam's club so i wanna have the whole cake now for july 4 to celebrate.so i wanna know, do i have to order the cake or can i just go and buy it.i was told that if it is not on the shelf then i DO have to order it.i just want to make sure.