Be advised that many (all?) online retailers offering digital downloads may not have serial numbers for them. For this game, all you are buying from the retailer is a serial number. The actually downloading of the game comes from ArenaNet. This situation should be temporary, but I'd be sure to talk to someone and verify (if possible) before purchase. Based on my situation Best Buy is (was?) selling digital downloads of GW2 but do not have serial numbers for it. Their problem was so severe, they may have temporarily pulled digital downloads off their virtual shelf.
Last I saw Amazon had physical copies, where I ordered mine. I would imagine that Game Stop does too.