how long does the sa could hold a bag for u?

In the event that my SA calls and I am out of town, I simply have her charge it to me ahead of time and then I'll come and pick it up. She'll hold things for 72 hours tops. I just figure that paying in advance is the way to go... that way, you're protected and it will still be there.
 
how could I pay for the bag? by credit card?
If so, then can i still get the tax refund when i pick it up at the boutique upon my arrival?

Pay with CC to hold the bag. Once you are in the store, the SA will prepare the tax deduction form for you to sign so you can get tax refund later. I think they can do that.