custom modified oscommerce site needs more changes

I need an oscommerce site which has already been heavily changed with mods and customised scripts to be changed further. The site is a curtains and blinds store which sells 'made to measure' and 'ready made' blinds. I need both of these categories to be displayed on the same page for each product, the link below is an example site of what we need to achieve: [login to view URL];rmwidth=45&rmdrop=130&width=&drop=&readymade=Get+Price%21 The current site has a back-end admin area where all price tables and sizes are maintained, then when a product is added by the store owner he selects whether or not it should use the price matrix table to reference the price or if it is a standalone product with a set price (for example 'ready made' blinds). Therefore when this change is made you will need to ensure that only where the same product is added which is available in both 'made to measure' and 'ready made' will the price calculator be shown. This description is only for the purposes of allowing you to bid a price and should not be used solely as the contractual deliverables as there is obviously more detail that needs to be thought out such as dependancies of other scripts, ensuring that everything is still working and that site transactions remain seamless to the customer, also the site is on a secure server with a certificate so any scripts that are part of the checkout process must remain secure and working without any unecessary messages. The changes need to be fully marked through the scripts so that any other developer can understand what the purpose of the module/function is. You are to use useful terms when naming functions (as per any good coding practise). You will only receive 50% of the money on the day of completion after I have checked that all seems to be working. You will then get the other 50% thirty days later after it has been fully QC'd in production. This is a small project but because of the trickiness of modifying oscommerce which has already been modified I am classing this as a medium project for budget...YOU NEED PATIENCE AND TIME TO TAKE THIS PROJECT ON. I expect you to be readily available by phone or MSN/ICQ for any queries, you should be able to respond to emails within 1 day. You should act in a professional manner and be accomodating to any changes required throughout the project. I do not expect silly prices, you will receive feedback accordingly.