It's been a while, but... I vaguely remember that the shopkeeper won't get surprised if one (or more) of your inventory areas (meals, snacks, toys, etc.) are full.
And, just in case how to shock him has slipped your mind...
Quoting from the V3 instruction sheet:
Did you find a secret code?
At the shop counter screen, press button (A).
Then, press button (A) quickly three times.
When the shop owner’s face appears surprised, punch
in the secret code to reveal a new item.
[Note: A = left button, B = middle button, C = right button]