Add simple cart/checkout etc to existing site

What I have: At moment all is manually emailed etc. Ready for a small step up to the next level (not huge traffic - 10-30 orders per month). Site has hardcoded product displays with "buy now" button that generates a mailto list. See attached jpeg. What I need: Add simple shopping cart and checkout functionality. Limited products (less than 15) no need for product database and cms other than for cart purposes (product display will stay hardcoded). I need to integrate this into the existing site using same basic backgrounds, styling, menus etc. Would like to turn "buy now" email creator button into an "add-to-cart" button. To acknowledge press by displaying dialog box over existing page (darkened) containing an acknowledgement (item name, pic, description) with box to change quantity/add to cart/cancel. (Ajax) Display "items in cart" / show cart on same shop page (eg above product display). On Show cart, new page - display cart, allow changes to qty or item delete with feedback. Cart item contains image of product as well as usual (qty, price, total). VAT calculation included in prices. On 'continue' get shipping options (limited list and one - regular mail - conditional based on which products in cart) - also might be able to get courier quote from delivery service provider by feeding address/postcode, service type and weight. Get delivery address details (add to separate customer table), billing address (for later activation, initial phase not required) - Ajax with javascript hinting and verify (similar to freelancer.com forms) before submission. Only single country and currency and tax rate for orders. On submit, email record to customer, to preconfig admin address and also store in mysql for web cms view/edit. Admin/web page CMS - view orders, assign/manage order status, add waybill refnum, update payment status, keep records and manage customer database, manage auto email addresses, shipping prices/options etc. Manage product database - only for cart display (name, pic, description, price, weight, mailable: yes/no). This will not be used for shop page product display as that will stay hardcoded (too few products to justify a system for this) No payment gateway yet (we will send electronic payment details with confirmation email) but need api hooks to facilitate later addition of credit card or other third-party gateway integration. (could be second project) Due to local suppliers courier options not very automatic but under investigation as to how to automate as far as possible via web post/lookup/get based on service type/weight/postal code. Your suggestions and input welcome on above. I really like to keep things very clean and simple, graphically rich, need html tags on all for css styling to fit site (I will do css). I did the website layout/design so can answer any questions. Restriction - I have only one allowed mysql database allowed and its used for forum and contact form already. All tables should have a set prefix to separate/identify them in the database. Provider also doesnt allow php mail, have to use PEAR mail. Timeframe for delivery: I would to have this up and running within 3 weeks.