We'll be using a pledge manager, so once the campaign closes you'll still be able to add on extra items as you want. If you're certain you'll want multiple items or multiple tiers, you can also add up the total pledge value now and plunk that down on any reward level.
Be aware that each country has its own rules and regulations about customs fees. Unfortunately we have no control over that, and we mark all packages at their honest value. This may mean your customs office will be jerks and assess you unpredictable fees to release your parcel.