You should not ship chicks at that age. You ship day old only birds or birds that are fully feathered around 8-10 weeks and older.
It is against postal regulations to ship them at that age. You could probably expect them all to be dead when they arrive.
ETA Postal regulations:
9.3.2 Day-Old Poultry
Day-old poultry vaccinated with Newcastle disease (live virus) is nonmailable. Live day-old chickens, ducks, emus, geese, guinea fowl, partridges, pheasants (pheasants may be mailed only from April through August), quail, and turkeys are acceptable in the mail only if:
a. They are not more than 24 hours old and are presented for mailing in the original unopened hatchery box from the hatchery of origin.
b. The date and hour of hatching is noted on the box by a representative of the hatchery who has personal knowledge thereof. (For COD shipments made by a hatchery for the account of others, the name or initials and address of the hatchery or the Post Office box number and address of the hatchery must be prominently shown for this standard.)
c. The box is properly ventilated, of proper construction and strength to bear safe transmission in the mail, and not stacked more than 10 units high.
d. They are mailed early enough in the week to avoid receipt at the office of address, in case of missed connections, on a Sunday, on a national holiday, or on the afternoon before a Sunday or holiday.
e. They can be delivered to the addressee within 72 hours of the time of hatching, whether the addressee resides in town or on a rural route or highway contract route.
f. The shipment bears special handling postage in addition to regular postage, unless sent at the First-Class Mail or Priority Mail prices.
g. When live, day-old poultry is to be transported by aircraft, all provisions of the airline tariffs are met and air carriers have equipment available to safely deliver shipments within the specified time limits, allowing for delays en route in air and ground transportation.
h. Day-old poultry, originally shipped by air express or air cargo and then presented for mailing, must be in good condition and prepared as specified in 9.3.2a. through 9.3.2e.
i. Boxes of day-old poultry of about identical size, securely fastened together to prevent separation in transit, may be accepted for mailing as a single parcel, if such parcel is not more than 100 inches in length and girth combined.
9.3.4 Adult Fowl
[9-8-09] Disease-free adult fowl may be mailed domestically when shipped under applicable law in accordance with 1.7. Adult chickens, turkeys, guinea fowl, doves, pigeons, pheasants, partridges, and quail as well as ducks, geese, and swans are mailable as follows:
a. The mailer must send adult fowl by Express Mail in secure containers approved by the manager of Mailing Standards (see 608.8.0 for address).
b. The number of birds per parcel must follow the container manufacturer limits and each bird must weigh more than 6 ounces.
c. Indemnity may be paid only for loss, damage, or rifling, and not for death of the birds in transit if there is no visible damage to the mailing container.
d. Postage refunds may not be available if the Express Mail shipment was delivered or delivery was attempted within three days of the date of mailing as shown in the "Date In" box on Label 11.
