I figured this out while trying to get rid of my coupon, oddly enough. It's not all that useful if you go prepared to buy everything in 1 shot, but it's pretty cool.
All you need is Jinsu Nogi v128 and the actual coupon.
1. Begin talking to corresponding NPC
2. Click "Shop"
3. Drop Coupon
4. Open the actual shop
5. pick up coupon
6. Before closing shop, drop coupon again
7. After closing, pick up once more
That's all. I thought this might help someone. That, and I wanted to post something "cool". I hope someone can use this, and I hope not to be flamed.
Thanks for reading!