Amazon has a Lightning Deals FAQ. Read it. Learn it. Live it.
"Yes, you can continue shopping and add more items to your cart if you like. However, we advise you to complete your checkout as soon as possible, and definitely before midnight Pacific time on the day you claim the Lightning Deal discount. Lightning Deal discounts always expire at midnight Pacific time on the day they are claimed, so you must check out before that. But there is also a possibility that other customers may buy our entire supply of that product at some time after your claim your discount and before you check out that day. In that case, you will not be able to buy the item and you will not receive the Lightning Deal discount. For that reason, we suggest you finish your shopping and complete your checkout soon as soon as possible after you claim the discount."