Trade-in Logistics & Store Credit

I found information about accessories, such as power cables or adapters, but it doesn't say anything about the original box. I am considering initiating a trade-in, and I did save the original box, however I won't be able to actually put my hands on it for a while, and I'd prefer to get the process started sooner. Is there any way to find out for sure if including the original box affects the valuation of my device?

I also read that the value of my trade-in could be used for a purchase on apple.com, but it wasn't completely clear to me exactly when those funds can be applied to a new purchase. I'll be just trading in my old Macbook Pro for a newer model... is that credit available to offset the cost of the new computer right away?