I think it's the 100wh rule. If the lipo you want to buy exceeds 100wh (total nominal voltage times amp/hour) than they won't ship it. This has to be verified but I also think that you can order one lipo that is under 100wh, or you can order 50 lipos that are each under 100wh and they will ship it because each lipo is individually wrapped.
You'll have to change your tactic. They won't ship a 5000 mah 6S lipo (111wh) but they will ship a 5000 mah 5S lipo (92.5wh).