This extension allow customers enter thier discounts coupons, gift certificates (gift vouchers) and reward points during the checkout.
Before this extension customers enter these codes only in the shopping cart page.
The module can activate via admin front from setting page.

• Allows customers to enter a coupon, gift certificates (gift voucher) and reward points during the checkout process.
  
• Admin panel control from store module page.
  
• Show / Hide if disable the coupon, gift certificate and reward points from the setting page.
  
• Auto hide forms if disable the status of coupon, gift certificate (gift voucher) and reward points from order total page in admin panel.
  
• Auto hide reward form if customer has not points or the poroducts in the cart didn't bought by points.
  
• Auto update the mini cart total in header.
  
• No redirect when applty discount.
  
• Auto width for the discount forms.
  
• Discounts box fully responsive.
  
• Auto update the mini-cart in header.
  
• Easy to edit for other custom themes if not support it.
  
• Support Ocmod.
  
• Easy to Install.

Please check the Readme_ApplyCouponVoucherRewardDuringCheckout.txt for how to install and configure it.
This extension will not replace any files.
Apply During Checkout files are separate from core files, other modifactions handled with ocmod.
Also, you can watch the video demo for more details.

Opencart 1.5.x :
please check this link : Apply During Checkout - Old Versions