This is the way it works. If you buy over the phone, usually they wont accept more than one voucher code per transaction. But if you go to store, you can stack them, I have used 6 vouchers at a time to purchase $300 item. If I return it for any reason, I will get the brookstone gc that never expires for $300.
By the way, put the vouchers in notepad or some app in phone, dont show them fine print. Tell them they are just gift cards.