We had a flower retailer some time ago running a different cart, but the following is what we did.  They needed the customer to be able to select a delivery date and time, but the availability dates would be determined by what time of day an order was placed as well as proximity to selected  holidays when flower purchases would increase. Significant custom logic was implemented according to rules the client established for us to code against.

With that said, if you could be precise in explaining what you need, it would help.  Your second sentence in the original post is missing a couple words and while I can fill in what I think you were asking, a more detailed request would be useful.  Thanks.

Clearly what you are describing is a custom scripting requirement.  This is very similar to the "flower delivery requested date" modification I described above, but as I say, this was performed for another shopping cart.  Same sort of concept though.

I'd suggest you check with a few ProductCart developers to see who could complete such a request within your timeline, and how they will approach the task.  Hamish, Dan from Netprofits and perhaps others post here frequently.  I'll drop you a PM regarding my own scheduling and other thoughts.