MAGENTO | Add checkbox before add to cart

WHAT NEEDS TO BE DONE: In our Magento store add a checkbox that needs to be checked by the customer before adding a product to the shopping cart. The checkbox should appear for certain products only. If the checkbox is not ticked then a pop-up message should appear reminding the customer to check the checkbox. GENERAL: Magento developers that have previously worked with the Zetaprints extension would have our preference. In order for us to keep track of all changes the developer will work in a test environment and deliver all modified files to us so that we can add/replace the files in the live environment. Proper commenting in files is much appreciated. We are looking for one or more Magento Developer to help us with a range of tweaks for our store. We post each of the tweaks as separate projects in order to make sure that the qualifications of the contractor match the specific task at hand, but we are as much as possible trying to establish a long term relationship with one or two all-round Magento developers.