Unless you are talking about pre-filled cartomizers buying prefilled is not very cost effective, as 1ml in pre-filled carts tends to cost 10-100 times more than in a bottle. Unless you toss each cart after the liquid is gone, you are going to be refilling anyhow.
If customs see carts, its been my experience they will inspect them. If there is liquid in them, they are not getting in. Much better, and cheaper to ship one order of empty carts, get them guaranteed, then gamble on the less expensive to prefilled, liquid.