Please follow the below steps and it should all work for you.
1. Make sure all the vouchers you want are available; you may need to go to an order in progress and cancel it, this will return vouchers to you.
2. Go to “buy node” pick the nodes you want.Scroll to the bottom and apply your voucher, add them by clicking ☑️ And choose what to oay in. At this point the vouchers values will be deducted from the order total.
3. Click on buy. If you have no balance to pay, your order automatically goes through.
4. If there is a balance after the vouchers are applied. Pay for the remainder of the order in your preferred currency. Make sure you send enough or it will become a voucher again.