If a product is not available, the system should allow for the user to place a link to a webpage for the customer to send an enquiry for the particular product.

So,when a customer see that we have the product but it is not available, they click on the link called, lets say, "enquire" and the system will take the user to an enquire screen, which we creatre within our website, for them to place an enquiry of when the product will be available.

This can be a fourth option under Shopping Cart -> Product Settings -> availability and call it something like: "no availability link" for the user to choose it and then ccreate the link to a website.