The first reserved is a verb and the second reserved is used as an adjective.
For the second sentence the action of reserving is finished, just the condition of being reserved is remaining for 30 minutes. The use of will indicates that the action will happen in future w
Assuming that this is some sort of order system that permits you to reserve an item while you arrange payment, I would suggest the following.
Your item is now reserved. Your reservation will be held for up to 30 minutes.
The person making these statements is asking you to take some additional actions while they hold the reservation. Therefore, the future continuous tense sounds