Modify Jigoshop plugin

Hello,

I've implemented an e-commerce site for my client on Wordpress with Jigoshop plugin. Have customized more or less front end code but now believe need a more experienced person to help me with it.

On the checkout page i have couple shipping options for customer to select. One shipping is Terminal delivery, this option requires additional action from the customer. The customer needs to select to which terminal his parcel should arrive. I have the list of available terminals, it should be a dropdown list (static list which I will provide) only visible when that shipping option is selected, also this field should be required to complete. If the terminal shipping option is selected and the specific terminal is not selected page should return Error on submit. Error check should go friendly with plugin's script and be displayed in the same box as usual errors.

If the budget indicated is too low, do let me know and I will check with the client to make it higher.

Thanks