I recommend calling the boutique in advance to check if they have what you are looking for. The customer service line has always been more helpful than some of the FAs I’ve encountered in-store.
Don’t be afraid to look around the store and ask about other materials, sizes, colours, etc. And don’t feel pressured not to walk out empty handed, when you know it’s the right bag, you will know! : )
P.S. I second your decision to go to MBS- I dropped by NAC the other day again and an FA gave me her card and told me to drop her a message about a bag I was looking for. I did and she never replied

I’ve had better service buying a mere Clic bracelet at Hermes.