can't you just set xp-amounts for lvls and another limit on when you get a shipment? I mean it's not like afther 6 shipments you lvl up, but more like afther xxx mount of XP right?
for example you could set it like this:
lvl1: 100XP
lvl2: 300XP
lvl3: 600XP
etc.
and then for the shipments:
one shipment afther 50XP
but hey I'm not making the game so, I've got no clue on how it works