We are working on a Webstore for which we've partially implemented payment via the Paypal site. This code needs to be tested and adjusted so that it works.
We also need to have PayPal Express payment implemented into the checkout sequence.
The store is built using an OpenSource CMS framework and implemented in PHP. We will provide a subset of the working store as well as a stub into which the PayPal Express payment option will need to be built.
Since this is a web application, the OS you're using doesn't matter. We have envisioned a budget of USD 150 for this.