Hello, a Gamestop employee here :)
While I am not sure about them sending codes in waves, there are times where our codes print on the receipt automatically and times where the employee has to input the SKU or find our backup codes in the search system. Majority of codes given out as a preorder premium automatically print on the receipt (this has been happening for more than a year, so I wouldn't be surprised if a lot of employees don't know it is possible to manually find the codes.)
Gamestop only runs out of codes if the particular SKU is removed from the system or if Gamestop actually tells the employee that they've runs out (in this case, the computer says something along the lines of "we have exhausted our supplies")
To have them find it, have them search for "backup xiv" in our system and add it into the transaction. If for some reason they can't find the code, have them input SKU 501972.
Good luck! :D