Add to Invoice

I would like the ability to select which reservations are added to an invoice.  Instead of adding via reservation date or date of entry, I would like to be able to pick and choose.  For example, with City League Teams, they pay for their match courts each quarter (in advance) and practice courts at the end of each month.  Since both are entered at the beginning of the year, I have to manually adjust each date of entry in order to invoice the months that have match courts.

It would also be very helpful to be able to preview an invoice before it's created.