Virtuemart multiple minimum order value

Hello coders, need one customization to be done: I have a online food order and delivery website, where you can order food and get them delivered to your home. I need the zip-code validation to be modified. Depending on the zip-code the user shoose (via defined dropdown-field) at the one-page-check-out, the minimum order value have to be changed. For example, you shoose the zip-code 9999, the minimum order value will be chanced to 15 USD. Or by choosing the zip-code 9998, the minimum order value will change to 10 USD. By chosing the zip-code, there have to be a message (in real-time) next to the zip-code field, like: The minimum order value is 10 UDS. In case, the user clicking "order now" without reaching the minimum oder value, a message have to appeare (in real-time), like: You didnt reached the minimum order value of 10 UDS. The site is using the one page checkout module from polished geek, VM 1.8 on Joomla 1.5.26. Doing your job well will mean many following projects for you :)