You shouldn't have anything to worry about. Black walnuts, as do a number of
other members of the genus, excrete a hormone - primarily from the roots -
which can hinder the growth of sensitive plants. English walnuts possess
this hormone in much lower concentrations and even less so through the
shell. And how large a gift box are we taking about ? It can't be too many
shells to begin with. If you have any concerns, use them for informal
pathways.

pam - gardengal

Use of Waste Products

Synthetic mixes may use organic ingredients that once were classed as waste
products - conifer bark and redwood sawdust. Although these particular
ingredients have been thoroughly tested for toxicity, pH reaction and
uniformity, other waste products also can be used."What comes out of the soil
should go back into the soil," say conservationists, but gardeners do not
always agree. While it is true that crushed almond shells, as well as grape
pomace (when composted) can be used safely as a soil amendment, walnut leaves
or crushed walnut shells poison the soil for garden plants. Sunflower seed
hulls look promising, but tests have proven that they contain some
growth-inhibiting properties. Any organic material, especially agricultural
by-products, must be tested carefully to ensure that it contains no toxic
elements.
Resource: The American Horticulture Society Illustrated Encyclopedia of
Gardening Ó1981
March 1991 Chemung
March 2000 Revised

Hi Pam --
Above is from Cornell site citing AHS's encyclopedia, but it's from 1981.
Thanks for your reply, helpful as always.
It was a 20 lb. box. I guess the quantity of the left over shells would
appear different to different people.
Thanks again