Mailslot
Catch bad US shipping addresses before the parcel goes out: local checks always, USPS checks with your own free USPS credentials.
Every returned parcel costs postage twice and a customer's patience. Mailslot looks at the shipping address when the customer presses Place order.
Everything checked lands as a note on the order for the packer, and the last 100 checks are listed for you.

Inside Mailslot
"Apt" or "Suite" with no number
* A **ZIP from the wrong state** (by the USPS national area its first digit belongs to).
USPS down? The order goes through with a note on it saying it was not checked.
Before you start
More for shipping & orders

Embargo
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Manifold
WooCommerce: ship one order to many recipients, with gift notes and slips