You can't combine shipping on items if you buy them at separate times unfortunately, so it can get expensive due to the high postage costs.
So if a box you want goes on sale you can either buy it right away and pay $30 shipping for just one box, or you can wait for another one you want to go on sale as well and buy them at the same time, but its risky since the first one will probably sell out.
IMO, Best option would be to get the boxes you want regardless of sales, then when you like something that is on sale, buy that with the others so you can save shipping.
If you are just buying 1 or 2 boxes, once you work out the exchange rate and shipping costs you will probably be able to buy them from a local hobby shop for the same price or possibly cheaper
Just fired off an email to dacardworld for specifics, to see if there's a way orders can be placed and held, so they can be collated at the end of the weekend and shipped for a single price.
Awesome hopefully they get back to you, make sure you pass on the message here!
No reply to my email or tweet
Finally responded to a DM
I would not hold them in your cart because the cart is not live and items could sell out. You would want to place the orders but put a note in the special instructions box to hold on processing until sale ends because you are placing multiple order.
Now I've got this somewhat cryptic response to my email: