Either way you have to communicate with the seller and I know about talking to him/her about a extension of the delivery time, but I don't get why Fiverr doesn't add the delivery time to each order like it would be a separate order.
It's just more work and more hassle to talk to the buyer and if you're unlucky he doesn't understand why or get's upset. If Fiverr would add the delivery time this would be one step less to worry about, because how on earth should I deliver more in the same amount of time given??