The complicating factors in my case are that I am not an "accredited
scientist" as FWS defines it & that there will be more than 7 shells of each
species, which is the mysterious cut-off number FWS adopted to determine if a
shipment is for commercial use (see 50 CFR 14.4).
 
The regulations in 50 CFR part 14 mostly refer to "accredited scientists".
For example, 50 CFR 14.24 says "..scientific specimens or parts thereof,
imported or exported by accredited scientists or accredited scientific
institutions for taxonomic or systematic research purposes may enter or exit
through any U.S. Customs port, or may be shipped through the international
mail system." This doesn't say anything about if amateur collectors may ship
specimens through the mail.
 
A.