This extension allows customers to enter their discount 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 and controlled via admin from settings tab.

• 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 form of coupon, gift certificate or reward points from the setting page.
  
• Auto hides forms if disable the status of coupons, gift certificates (gift voucher) or reward points from order total page on admin panel.
  
• Auto hide reward form if a customer does not have any reward points or the products in the cart can’t be bought by points.
  
• Auto update the mini cart total in the header.
  
• No redirect after applying any discounts.
  
• Discounts box fully responsive.
  
• Three types of forms display.
  
• Enable / Disable apply button custom text.
  
• Auto updates the mini-cart in the header.
  
• Easy to edit for other custom themes if not support it.
  
• Support Event System.
  
• Easy to Install.

Please check the Readme_ApplyDuringCheckout.txt or read it on documentation tab for how to install and configure it.
This extension will not replace any files.
Apply During Checkout files are separate from core files, other modifications handled with events system.
Also, you can watch the demo video for more details on how to install and use it.