Shipping usually has a base charge, then an incremental charge based on additional weight.
With that in mind, the more you buy (weight-wise), the cheaper shipping becomes per unit weight. So the sky's the limit. Or, more likely, your wallet.
Seriously, though, I usually build up a wishlist of stuff and then place an order every few months or so after I've built up a sizable amount. Unfortunately, buying a lot of samples doesn't help much because they don't weigh as much (as a tuo/brick/beeng).
Another option would be to combine orders w/ other people. However, unless if they're nearby, you'd still have to re-ship once it arrived (which would probably erase any potential savings).