Its alright I just want to help. The bag will keep the leaves from drying out, like lettuce would if you left it on the counter. You don't want it sealed totally, just enough to make a mini greenhouse of higher humidity to try and save the leaves because they become damaged and crispy when they dry out to a certain point. It would not really be a big deal actually if the tree loses them, but would be better if it can keep them long enough for their stomatas to adjust to the new conditions and toughen up. You could also try misting a few times a day and see if that helps.
That is a big jump from a shipping size plant, 15 gallons is a big pot and to fill it the tree would end up being like 6' x 4' so you might want to think about downsizing now before it grows new roots it if that would be a problem. A container that big is going to be tough to water correctly inside... I don't have much experience with that but you want a long chopstick stuck down into the mix that you leave in (must be in for at least a couple hours to pick up the moisture) and pull out and feel it against your wrist for moisture, don't water until it is dry 4-6 inches down. To wet the mix evenly you need a larger volume of water, depending on how much the tree is drinking (not much right now with few leaves) and how dry the mix has become. Small amounts of water cause wet and dry spots that are bad for the roots. If the potting mix was moist and the chopstick says wait then wait, if it is loose and dry then your whole problem could be dry roots. Make sure the container can drain and the excess water is not sitting in a tray underneath.
There is a houseplant forum on Gardenweb where you can also get advice about the finer points of growing plants inside.