Tick Box required for website payments page for customers

I require a module or code designed and installed on my site for customers to select (tick box) at payments checkout page which will also appear on their order confirmation form on completion. the tick box will be for customers to approve an alternative costume if they one they have selected is currently not available. But then this should appear on their order confirmation as proof and / or verification that they have done this. There needs to be a record on file that the customer has selected their option. I have attached the payments page of my site to where I want the box to appear which is the payments page, with some other small changes required. They will be required to select a box Yes or No prior to proceeding to the next page for payment. FTP details will not be provided only to the successful bidder. Works are to be completed within 2-3 days if possible. FULL payment on completion of works, NO milestones will be given. All works need to be completed to my satisfaction prior to making payment. I am an honest employer as you should be an honest employee. ENSURE YOU UNDERSTAND THE PAYMENT PRIOR TO BIDDING. No Newbies only experienced developers with high feedback. I am using Cubecart V4 as my ecommerce therefore ensure you know how to use this package. No alterations to my site is permissible. Only changes to the payment page will be accepted and the order confirmation page. If you think you can do the works, please place your bid honestly. I am on a tight budget therefore can not offer too much, but if works are satisfactory I will also give more work out, I am always updating my website.