The limit per transaction for Disney Gift Cards is $2500 (max $250 off using the GCWEEK10 discount).
However if you're buying the giftcards dotcom "in store" gift card for 10% off first (using promo code GIVEGC10), then you are limited to 3 per transaction and it seems like you can do 2 transactions, so a total of $600. Since giftcards dotcom ONLY sells gift cards, their "in store" gift cards are usable on Disney Gift Cards. So it stacks with the 10% off Disney Gift Card GCWEEK10 promotion.
The easy route is to use the GIVEGC10 promotion for 10% off, plus shipping. Depending on how much you buy and how much you pay for shipping, this should net you somewhere around 9-10% off face value plus whatever credit card rewards and discount you might get from Chase offers (I finally see the giftcards dotcom Chase offer on one of my Chase cards this morning ugh). This is also limited to $2500 per transaction, so it is easy to get a lot of Disney gift cards this way and with very few cards, so less combining.
The more complicated route is to stack the $600 in store GCs and use them to buy DGCs. You pay $1.99 shipping for each ~$109 DGC, but your net savings is about 17.4% off face value before any credit card rewards, Chase offers, etc. You also run the risk of losing one of the gift cards in the mail. It is crazy that they don't sell Disney e-giftcards, I wish they did! You are also limited to ~$650.