We have developed a tool that enables any e-merchant to offer a "try-before-you-buy" option to their customers. To be concise, customers can order without advancing money, be delivered and only pay for what they decide to keep.
We would like to integrate this solution completely in the purchase process as a delivery method or as an option they can choose in the cart directly (or even as a payment method).
We have seen that the checkout pages are not customisable (excepted when Shopify Plus) and that best way to do that kind of things would be to develop a module that is downloadable on the store and so it could be easily integrable for the e-merchants.
How should we process? Do we need an "agreement" from Shopify? How could we get it?
You really should contact shopify first and work that out