1) Add one additional Tab at the Magento checkout page.
2) Selected items is listed at this new tab. If the customer has chosen the sale item then the shopping cart should be updated with this new sale item.
3) These sales items can be selected manually at the backend of magento, whether it should be listed at this new tab.
4) This extension should not have any bad impact on other currently working extension.
For more detail information please see the attachement.
Magento Version: 22.214.171.124
If you have any questions please do not hesitate to contact me. Thanks !