You have to dot your Is and cross your Ts. Chances are there was some infraction. Were there 3 copies of the invoice? Did you include the buyer's phone number on the custom's form? Was it a dutiable item?
"Observations
First-Class Mail International and First-Class Package International Service items may not contain dutiable articles.
Commercial invoices are required for all commercial shipments regardless of value and for gift parcels valued at $50 or more. No invoices are required for gift shipments valued at less than $50.
For all shipments valued at $50 or more, the sender should submit to a Salvadoran consulate copies of the commercial invoice in the form prescribed by the Salvadoran regulations. The consulate returns 2 legalized copies to the sender, who should mail them to the addressee direct, not enclosed in the parcel.
For commercial shipments valued at less than $50, 3 copies of the invoice (which need not be legalized) should be mailed directly to the addressee.
Certificates of origin are required only if exemption from or modification of customs duty is claimed under a trade agreement.
Correos de El Salvador requests that the mailer place the addressee’s landline or mobile telephone number on the customs form for each trackable item (i.e., an item sent with Registered Mail service, Priority Mail Express International service, or Priority Mail International service). This contact information assists Correos de El Salvador in calling or sending a text message to the addressee when the item has arrived."
