Custom shopping cart

We need a custom shopping cart developed for quantity-based pricing on products. There are a few reasons this needs to be a custom cart, and not a modified out-of-the-box cart system. There will be well over 5000 products (hand-entered during development to find bugs) in a few hundred categories (recursive from 20 +/- main categories). The required variable option pricing is unique for a few reasons because there are 6 ways options can affect per-item prices, per item, a flat rate, and a percentage (based on list per-item price). These options need to be calculated in a certain order to ensure pricing is accurate before any item is added to a user's cart. The options also have to be dynamic in terms of what input types is displayed to the end-user allowing option selections or text-based entry (checkbox, text, textarea, select, hidden, radio). There are minimum quantity requirements for all items. Some items can be ordered in lower quantities than the listed minimum, but additional fees are imposed for doing so. Some item prices includes shipping, some do not. For those prices that do not include shipping, weight-based calculations need to be during the checkout process. We also require that the interactions with templates, databases, cookies, and sessions be object-oriented (classes). Reporting features are needed to show trends such as which products are popular and which are not, and which items are viewed the most or ordered the most. We also want to be able to peer into temporary shopping carts so we can see what people are checking prices on before they decide to purchase. The payment system will need to be interfaced. We intend to accept credit cards and eChecks online with this system. Other offline payment methods such as purchase orders, paper checks and money orders will also be accepted, so some facility to update order status when such payments are received will be necessary. After a customer submits an order, they will need to be promted for file uploads (artwork) that will only accept a predetermined list of available file extensions. This list of extensions should be updatable from the administration area. We are open to suggestions on how certain features should be implemented, but already have a good idea of how the finished product needs to work for our administrators and customers. I need this as soon as possible may be with in 2 weeks